Mesa has been about 5 years behind OpenGL, seems this follows the trend, not sure if that's a good or a bad thing. After all it's not falling behind but it really doesn't seem to be closing any gaps either. So 4.0 is DirectX11 generation hardware, CodeWeavers have said they hope to have DX11 support in WINE within a year. That would be nice, several games I play that are no-go in WINE and would be at least one obstacle in going back to Linux.
If you are playing games with Mesa you are doing it wrong. You should be using the native OpenGL driver for your graphics hardware. Mesa is a software reference implementation/compatibility layer. It allows drivers with missing functionality to fill in the gaps and for ports of OpenGL to other hardware to see software implementations of the spec for comparison. Also, for systems without dedicated graphics it can provide software level OpenGL functionality but you probably wouldn't want to play 3D games on those platforms. Its very useful in the graphics ecosystem but probably shouldn't be directly used by end users looking for the highest performance their gaming system can offer.