Now to address your specific questions:
Is there a noticable performance difference?
Not really, no (some details)
Depends on the platform, and no this doesn't really help modability
Is it possible to use libraries developed for one language while developing in the other one?
I think so (although I've never needed to do this, because all the libraries I use were programmed in C#)
Yes (although I try to avoid it, for consistency reasons)