TechPowerUp's
R9 270X reference card tests found peak power usage of 172W, not far off from the rated 180W TDP.
A test on a
Sapphire R7 265 (the former 7850) topped out at 132W. (Rated TDP is 150W.) Considering the fact that AMD is usually very aggressive with their voltage and that Pitcairn and Tahiti can often save power by undervolting (even in 100%, 24/7 loads like cryptomining), it's certainly plausible that AMD could get the power usage of Pitcairn to the 110W-130W range, especially if it's only running 1024 shaders instead of the full 1280. But that's not going to be a competitive product at anything above the $150 price point, considering it won't be a modern card with modern features. More than one full year ago, that same TPU review described the R7 265 as being "
based on the aging HD 7850". And this was before the release of GM204 and GM206.