I have had experience with 2 GTX 1080 FEs in both air and water, and my findings are as follows.
The factors in sustaining a high clock depend on the following.
1) The GPUs capability on a per voltage level. i.e. what can it achieve at 1.0V 1.2v 1.4v 1.5 1.6 and so on. Using the typical OC utility where you just set +100 or +200 will apply that amount to the OC on all voltage levels.
With my card, it could maintain a +200 on all voltage levels above the stock curve, but can also achieve a +226 above stock curve on voltages above 1.04v. To obtain that level you need to use a utility like GPU Boost.
2) To maintain a certain voltage level, you need to be within TDP. With the FE its 120%, if it raises above 120% it drops a voltage level and will lower your clock speed.
3) To also maintain a voltage level and max boost you need to keep you temperature below 40C. Over that and your Boost will drop a certain amount for every x degrees it goes beyond that.
With air cooling, it is impossible to keep the card cooler than 40C and while you may initially see your overclock reach 2050 mhz, the hotter your card get the lower your clock speed will go. With my fans on Max I was able to hit 2050 mhz regularly, but not all the time. It would fluctuate depending how hot the GPU was.
On water cooling I can maintain a constant 2101 - 2126 and only dip if the TDP raises above 120% at a certain voltage. Since I have capped my boost at 2126 from a voltage of 1.04 and up I will see it drop to 2101 mhz if the TDP requires the voltage to drop below 1.04. My temperature never goes above 36C.
In an ideal world, you want to get a great overclocking card with a TDP much higher that 120% and stick a water block on it. You will then easily be able to maintain the maximum boost permanently since the temperature and power limit will never be reached. You will then only be limited by the max overclock the card can do at each voltage level 1.093V being the highest.
My other GTX 1080Ti can only do +150 on water, I haven't done much testing on this one, but without using GPU boost and under the same conditions as my primary card it struggles hitting 2101 mhz.
This is what i've found, and have learnt from. In future I will only by AIO extreme units that have a higher TDP and water-cool them.
