The 6800 XT has a TDP of 300 W. The 7800 XT is 263 W. The 9070 XT is rumoured to be 220 W. Performance has slightly increased, too. I don't see how this isn't an improvement in efficiency.
The 6800XT:
Process 7nm
Transistors 26,800 million
Density 51.5M / mm²
Die Size 520 mm²
The 7800XT:
Process 5nm
Transistors 28,100 million
Density 81.2M / mm²
Die Size 346 mm²
The 9070XT?:
Process 4nm
Transistors ???
Density ???
Die Size ???
I don't believe what's listed here makes for valid comparison but I'll go with it.
The jump from 6800XT->7800XT looks like a die shrink with minor improvement.
It's rumored that the die size of the 9070XT is significantly smaller. Like
really small.
Suggested between 7600XT/7700XT die sizes. Probably in the realm of 262 mm²
Performance estimated to be like one of the 7900 cards. The size of those dies are 529 mm²
Soooo a 50% reduction in size on a new process with x number of transistors and better density.
We need more information because the potential is all over the place but I'm not expecting miracles.