It's not impossible for the PS4 to use Tahiti since GFLOPS/W performance nowadays is some 16:1, so the card itself shouldn't draw more then 130W on full load. At launch both the PS3 and the X360 drew >300W each for the whole system and I don't see the remainder of the Durango/Orbis system making up something even close to that number.
Of course it would be easier to use the Pitcairn core but maybe the architecture is gimped in some aspect Tahiti isn't, or maybe Sony discovered it's going to be cheaper to use the bigger chip and have a much greater wield per wafer due to all the cut components.
Anyways while maybe there are some fanboy dreams from one side there's also PC supremacists and Ninty fans in the other doing the same job...
Essay Pro