I seriously doubt that current games can be ported to "managed" code without performance loss: it would result in a game with a very low FPS and therefore not be playable. Games are performace based, and (native) c++ is faster then .net c++, as you can't
tweak .net code that much as you can do in c++. Thought, it could work on some higher level, so a mix could work, I think. But isn't your footprint larger if you mix it?