It's really not that bad and yes I spent a couple of hours playing BS:I yesterday on my core i7 nVidia 660 Ti gaming system with all settings set to Ultra. My AMD system is a Kaveri APU based system and lo and behold, the only game that requires very low settings is BS:I. As I understand it, BS:I and the other game he mentioned are using some form of emulation, similar to WINE for the game to play, this is true of the Witcher and will probably become more and more common.
So, ONE of my steam games plays better on nVidia than AMD, admitedly, I only have 24 Linux/SteamOS games but I tend to stick with the high profile shooters but one game plays bad and Michael L. makes a big stink. When my Kaveri came out, all the comparisons were against the top of the line i7 and i5 processors and it looked like crap. Using a car analogy, a Camry with a V6 compared to a Hellcat Challenger will look pretty slow but for all other purposes the Camry will have more than enough acceleration to satisfy the average driver.
Hell I'm just happy the games are coming to Linux, whether the run perfectly or not, I'd rather play on Linux in low settings than Windows in high.