The performance per Watt flip-flops all the time between generations, it wasn't that long ago that we had the 5870 being much more power efficient than the 480 etc.
The thing that I disliked about the 290 I had wasn't the fact that it was power hungry, but that the reference cooler sucked big time, AMD do need to up their game as far as reference coolers go. (That coupled with the fact that it suffered blackscreens due to cheap memory but thats another issue)
That said I really hope the 4GB rumours turn out to be false, as it really would put me off, I want a single card to use @4k untill the next gen die shrinks, at the moment I think it's going to be a bit of a psychological 4GB barrier that shoves me in the direction of the 980ti next week when I make the decision.
Hopefully it at least equals or beats the 980ti at 4k forcing a bit of price competition.