I only log in here when gaming is quiet so this is the first time I've seen the message. I'll outline some of my adventures with the FE below and perhaps there's something in there you find useful. It's a more fussy card than any I've previously overclocked and my insight should probably be filed more under alchemy than science.
As a general proviso to what I'll say next, I'm into overclocking but only to further my in-game performance. There have been battles won and events recorded in Eve Online's history on the basis of, say, having tri-SLI TitanXs versus not having them. It's less of a factor today in that game but this kind of thing really happened. So I'm less concerned with power draw in 3dmark or Superposition compared to power draw in a 3000 man MMO fight with all details on, 12k resolution, etc.
Therefore I'd muse that it's possible that your card will also pull the same power in that specific instance, given that the 1100mV of an FE isn't likely to push 600w in Port Royal, and presently you can't solder an Elmor anywhere to change that (that I know of anyway?). We have seen some examples of extreme power draw in Path of Exile, for example, with the highest shadow setting and quirks like that.
Another thing I'd point out is that I only bench with the settings I game on. So I launch 3dmark through Steam, keep textures on 'Quality' in CP, etc. Doing a run like that just now on Port Royal, gives 15457 (
https://www.3dmark.com/pr/841767 ) and I don't think it exceeded around 600w draw.
Which leads to the next thing, before I talk about modding, which is how power is measured.
I have two means: one is to run CPU/board on a different PSU, and then use my Asus Thor 1200 to monitor at least the 8pin connections (using its OLED display) - though of course this doesn't include the PCIE power.
I had to abandon this method because my shunt mod somehow stopped the Thor 1200 working properly - it randomly trips OCP on the card (despite being nowhere nearly total OCP, which is in fact way above 1200w on a Thor, and we're not close to 1200 anyway). I switched to using an EVGA 1600 T2 and this solved the problem completely (probably due to the analogue controller); I haven't had a crash in weeks on my 3090, even benching. I value stability above everything else (you crash at the wrong time in games like Eve, and you might lose more than the value of a 3090).
So with the 1600 T2 plugged into a wall monitor, my other method is to compare the load to an unmodded Titan RTX (my modded one messed with psus as well but that's a different story). I know the peak draw of the unmodded Titan is 320w. So let's say a given test pulls 550w total system power on the Titan, and at that moment the Titan is 100% load and showing 320w in AB, right up on its limit. If the 3090, in the same scene, is then pulling 800w from the wall, we can say that it's pulling 250W more than the Titan, so 320+250 = 570w in that specific example. Of course, we also have to acknowledge that the slightly higher fps of a given test under the 3090 might mean marginally more work for the CPU - however, I don't think that increase is likely to be more than 10-20w, and that's generally within the margin of error of the wall monitor anyway. Even though we're measuring from the wall, PSU efficiency doesn't play a major role in this since we are comparing like with like - arguable minor variance in PSU efficiency across its power curve might affect our results slightly, but again I'd argue this to be less than 10w.
As for modding:
Initially I shunted the 8pins and left the rest alone, as I used to on previous cards. As you'd now expect, this didn't produce great results - the power stayed around 400w, and SRC showed odd values.
Then I decided to shunt all the 5mohm resistors (by stacking with 5mohm) though this led me to be concerned about the PCIE current. To combat that at little, I'm using EVGA's old supplemental PCIE power plugs, to reduce the trace distance across the length of the board from the 24 pin to the PCIE slot, the journey along which I'd expect problems to occur if we're exceeding 100-120w draw from the PCIE.
This produced a (maximum) power draw that my above method estimates around 550-580w. It would tend to cap these kind of values even the the tdp target wasn't exceeded, which goes along with what you mentioned about hidden limits.
Around the same time, I had the problem that 8pin #1 was reading a lot higher than 8pin #2 (8pin 1 being the left one of the shunts when viewed from above with the display port facing left). This is a really fiddly place to solder and drove me mad trying to get it right, so I swapped that with a 3mohm, and for some reason also swapped the one below with a 3mohm (which on multimeter shows continuity). This helped balance them a bit but still somewhat odd results.
From somewhere I had some tiny 8mohm resistors in a smaller package than the usual ones, so I put these onto the tiny shunts nearest the core, relatively speaking (even though the size didn't match perfectly, making it messy). I'm not sure if this truly did anything productive or not and at the time I also changed the top left main shunt on the back (displayport facing right) with an 8mohm, for reason I can't remember (possibly red wine).
Anyway, around this time, PWR_SRC started sometimes reading 0 (although I have heard on here about that happening to others too) and an odd thing would happen often, where the card would just get stuck in 'IDLE' even under full load. I took screenshot the first time it happened (still on air cooling at that point):
http://rooksandkings.com/mini/idleload.jpg
So you can see the card in sections there is pinned at 1100mV, running full clock and load but stuck in performance mode 'IDLE' rather than vOP/ReL/PWR or TMP. In this mode, using the above methods, I measured it pulling 700w+ from the wall. As you can see on the timeline, I was puzzled enough by this that I started it up again and as you can see, it ramps back up to 1100mV and full load while staying in IDLE, not vOP/REL. It also holds the 'Effective' clock very close to the display clock in Tempspy.
Some other points: I can't run this card on an Asus Thor 1200. It'll trip after an hour or two. I've run it for weeks on an EVGA 1600 T2, no issue, 100% stable with 2200mhz on the core now that's on a loop. Sometimes it runs on vREL/OP but it does usually hold 1100mV.
Now, if you want to *really* go down the rabbit hole here's something absurd though I can't verify it: after explaining this to a friend, he was (for other reasons) repeatedly opening and closing HWinfo, Tempspy, afterburner, precision, and at at some point his unshunted FE got 'jammed' and stuck in idle under full load, where it appeared (alas he can't measure wattage but judging by a bench he ran) to be drawing well above 400w and holding 1100mV. After reboot it couldn't be repeated, no matter what he tried. Make what you will of that part - I have no idea on that one; I modded my card in the launch week for 3090 and can't test that. Still, part of me believes there is a way to trick the entire monitoring system of these cards (even unshunted) and essentially use the IDLE mode to lock 1100mV and draw whatever consequent power that entails.
I can assure you that I'm no soldering wizard but if I take the block off again (Corsair block - all I could source up to now) I'll take you a PCB picture of the mess. Normally I'd offer to do it just to indulge your curisosity but the stacked shunts initially made - I think - contact with the block, needing me to insulate the tops against a potential short and generally it took me 3 attempts to get the block on satisfactorily and I'm scared to take it back off except to change it to a better block (even a good loop is only giving me 35-40C or so).
I doubt any of this is particularly helpful because it's as much alchemy as overclocking. I've never had a board that has such odd power balancing and although I love the peformance generally, I can't pretend to properly understand it. If I can give you anything, it's my belief that there's some kind of way to trick this board using the 'IDLE' power mode and if I ever figure it out I'll certainly post it up.
Anyway, sorry for the delay and the long essay - I realise that 'adventures with a pcb' makes for very specific bedtime reading