I am perfectly happy with my 6900xt. Running it at 220w limit which still manage 20k graphical score in Time-spy. The trade-off was 10% performance vs 350w for 37% power reduction and roughly 5% performance lost vs 300w for 27% power reduction.
Previously was running tuned for 300w but have since switched to 220w for good. Performance lost in game is not noticeable but the reduction from 220w-250w max power draw to 150-170w is huge. PC is completely silent and i can game alot longer without having to turn on the AC.
7800 XT is a good purchase since it is pretty much a better version of 6900xt out of the box. (Not comparing with those xtxh 6900xt that are throwing away power efficiency for that additional 10% performance vs running at 350w limit.
Tomshardware's GPU Hierarchy has the 6900xt being around 3.2% better in gaming performance (FPS) with 1080P Ultra. 5% in 1440p Ultra. Raise the power limit on the 6800xt and overclock and it is basically a 6900xt.
That was also why 6800xt was the best price/performance and the most popular card of the 6000 series.
There are 6900xt with xtx and xtxh die with the latter being in another class of its own. The xtxh 6900xt offer the same level of performance as the 6950xt and those are the only 6900xt that were worth getting over a 6800xt.
So is this worth it for an upgrade to a 2080 super? Or should I find a used 6900xtx? Or wait for the 8000 series and grab a 7900xtx used when they drop?
Even if you somehow find an used 6900xt that is cheaper you are giving up better rt performance, new features, longer driver support, warranty so imo i don't think that is worth over getting a brand new 7800xt at its current price point.
Waiting is also an option. But it is hard to tell with AMD not competing for the high-end market with their 8000 series. Owners of 7900xtx may just opt to wait till 9000 series to upgrade.
Morepowertool version 1.3.9 is what you are looking for. Unfortunately it won't work with 7000 series and future AMD cards due to AMD decision to lock down bios.