is DX11 a gimmick on current HW implementation or what?
Yes, I agree that ATI 5XXX are faster then 4XXX and faster then current NV cards.
But, based on AvP the DX11 is useless - not much video benefits, but a lot of FPS penalty - isn't it?
Yes, ATI 5XXX is the best you can get for now, but doesn't look like it is ready for complete DX11 support power wise.
Unless AvP DX11 was implemented incorrectly

that I can NOT say for sure or even at all.
Are there any ATI / AMD comments about DX11 performance metrics?
May be ATI just short cut DX11 implementation to release cards, where NV went all the way through for full DX11 acceleration, but got into implementation / production issues? Who knows?
Looks like 2 diff strategies. Time will tell which one will really deliver DX11.
Just my observations / speculations.
