Back in the day, w/ AMD CPU and Nvidia GPU, I was gaming on Linux a lot. ATI was basically unusable on Linux while Nvidia (not with the nouveau driver of course), if you looked past the whole kernel driver controversy with GPL hardliners, was excellent quality and performance. It just worked and it performed.
I was playing World of Warcraft back in the mid 2000s via Wine on Linux and the experience was actually better than in Windows. And other titles like say Counter Strike 1.5, 1.6 and Q3 of course.
I have not tried that in a long time. I did hear exactly what you're saying here. Then again I heard the same about AMD buying ATI and things being OK now. My other reply(ies) elaborate on what exactly the experience has been if you're interested.