For Nitro+ 6800XT you have GPU power limit set to 290W by default compared to 255W on reference design card.
This mean when you apply 15% powerlimit you allow 332.35W to GPU core. You need to add to that around 45W for the other components of the GPU which means you have a total of around 377W.
This is close to the limit of what 2×8pin and PCIe slot can supply a combination of 375W. You are driving your card on it's limits.
Maybe you need further tuning. I thought Nitro+ can achieve better results than reference cards with respect to overclock and under volt.
A good air circulation hopefully will help with the temperatures.
I have Cooler Master Cosmos C700P which provides a lot of air flow with 2 fans at bottom for air suck in and 6 fans on radiators that push the air out.
Before I went water cooling for 6800XT, my temperatures were 67°C and junction 90°C max under stress tests. At that time, I had modified fan curve to go to 100% fan speed relatively fast.
I have water cooled setup now. The 6800XT has maximum 47°C and Junction of 67°C in stress tests but Junction temperature in games is around 54°C.
I have reference XFX 6800XT and I use 275W for GPU core and 305A TDC applied by using MPT additionally I have 15% power limit in Wattman.
This means the GPU core is allowed to take 316.25W and total GPU power is around 361W.
I am able to achieve 2580-2612MHz clock in games like Assassin's Creed Valhalla.
My setting in Wattman are 2650MHz@970mV.
These are my settings for daily gaming and not just to achieve highest score for short Benchmark.
I am achieving a score of 19376 in Time Spy with this.
So far very stable in all games that I recently played (Borderland 3, Assassin's Creed Valhalla, Cyberpunk).
Mine does not like VRAM overclock though. Even 50MHz increase on MCLK causes crashes in benchmarks. Maybe it has to do with SAM. I have not tried it without SAM.