If the drivers are crippled, why how is it that there are no performance differences. Intel may have the worse drivers but at least they don't cripple one line for the sake of another.
In any case that driver crippling shouldn't be as big a thing in OSX nor how bad Intel drivers are. It is down to Apple to at least a significant degree. They came usually late to the party with API support anyway. I don't know if anybody ever bothers with actually comparing the pro performance in OSX with serious benchmarks.
The Windows side only say what it can or should be. Difference between Intel and Nvidia drivers in OSX might be much different. For the HD 3000 drivers I think the OSX drivers were better than the Windows ones.
In pure raw power the Intel HD 5200 is probably a match for a 650M and especially in pro software having that lower latency memory access with the big L4 Cache might sometimes be worth a lot. If the drivers are good I think that GPU should be capable of quite a bit.