Running Witcher on Linux under Wine works pretty decently, but there are some bugs. For example some models are distorted (like fingers for example), as well as the notorious blank health/toxicity bars bug. So far I didn't find solutions for those. Did you encounter some bugs which you managed to resolve?

I didn't think the Linux video card drivers were really up to doing this, are you running it on super low settings?

Nvidia closed driver is more than enough if you have a good Nvidia card. You can use the high settings if you card is up to it. Nvidia driver shares most of the code across different systems, and uses a system specific layer on top of it. Some overhead comes as a price of Wine's translation of DirectX calls to underlying OpenGL ones, but it all performs surprisingly well in the end. For anything besides Nvidia I can't say, since nothing really competes with their driver in OpenGL support as of now.