Seems I went too low on my first undervolt so I increased voltage from 850 to 900 and now my 3070 gets 1935mhz (1920mhz at stock), runs at 65 degrees (75 degrees at stock), power usage has gone down from 240w to 180w and my FPS (and superposition bench score) is 5% higher than stock settings.
What surprises me is that Nvidia recommends 750w PSU (for the 3080 anyway) when this is completely unnecessary, the cards are just poorly optimised. Many people probably went out and bought new unneeded power supplies
I think it's an understandable caution on their part. When nvidia (or any other company) gives recommendations, it will be for the worst case scenario plus a sizable safety margin. Same goes for default voltages and pretty much everything else. What you're getting is default power settings that the manufacturer is absolutely certain will work reliably for every single card they make and a recommended stated PSU wattage that the graphics card manufacturer is absolutely certain will work reliably for every PSU and every reasonably plausible PC configuration. The lowest quality PSU labelled as 750W (which in reality will probably be straining at its limits at somewhat less than 750W) and a CPU with a relatively high power draw and 6 fans and 4 SSDs and 2 HDDs...that sort of thing.
Imagine you're running a graphics card manufacturer. It makes umpteen thousand graphics cards of a particular model using a particular GPU. There will be some minor variations in manufacturing (inevitable given the miniscule tolerances involved in a graphics card), so the voltage actually required for a specific card will be (for example) somewhere between 860mV and 930mV, with most cards needing somewhere between 880mV and 900mV. So you ship all the cards set to something over 930mV, something high enough to ensure every card you ship will work. Maybe 975mV. As high as it'll go without running into significant problems. What you're shipping is one setting at which every card will work reliably, not custom per card settings at which that specific card will work most efficiently. At most, you might test after manufacturing to pull out some cards that will work reliably at lower voltages so you can change the settings a bit for those cards and sell those cards as a factory overclocked model.