nikoli707 :
amds tahiti chip in the 280 definitely outperforms the gk206 in the gtx660. not sure where your finding any benchmark of the reverse. generally speaking the 280 is on par or slightly faster than the gk204 gtx760(and gtx960) and slightly slower than the gtx670.
An R9 280 can absolutely render more detail or more FPS (or some combination of both) than the GTX660 in GPU bound conditions in any game. I'm not claiming anything to the contrary, you're just misunderstanding what is being said here.
Render throughput is irrelevant in compute bound conditions. The GPU can't do the CPU's job. Nvidia's DX11 implementation is better optimized, and in effect, lifts the performance floor in compute bound conditions. You'll have to get your head out of the GPU-bound bench-marking bubble to understand this. It doesn't matter if it's a GTX460 or GTX980, either one is capable of higher FPS minimums in WoW than any AMD GPU, period, as FPS minimums in WoW are hard limited by the CPU, not the GPU. The Nvidia DX11 implementation is more forgiving in these conditions.
In GPU bound conditions, visual quality and FPS are inversely adjustable, GPU "performance" does not set hard limits on FPS as it can always be adjusted.
I'll try to give you an example:
Hypothetical R9 290 vs GTX660 in WoW, all settings equal (lets assume 1440P ultra), all other hardware the same, same conditions in game...
In low traffic areas of the game, when the R9 290 gets ~140FPS the GTX660 will get ~80FPS. This is a largly GPU bound condition, as the compute workload in low traffic areas is low.
Join a highly congested large raid, and the compute workload will increase dramatically, the bottleneck will shift to the CPU. Conditions that drag the machine configured with the R9 290 to 30FPS, will run at near 40FPS on the machine configured with the GTX660 because Nvidia's DX11 optimizations allow the compute workload to scale to more threads, making better use of available execution resources on modern CPU's. (imagine having a "mantle" setting for WoW, well, Nvidia's DX11 implementation like a stop-gap to the lower overhead API's of the future).
So, what's better for WoW? 30-140FPS? or 40-80FPS?
So yea, the GTX660 is in fact, superior for WoW than any AMD GPU, as it runs an API implementation that results in better average performance for this game.