Um no. 10% to 15% maybe for unmanaged C++ vs C#. And that's assuming you're taking significant time to optimize your code, which most developers don't. The biggest trip up with C# for developers for games specifically is managing the GC. If you can manage that, it should perform amicably. And of course you can use both if you want in a project.Since the modding API was first announced, they said it was in C#. It would be utter lunacy to switch an entire project from C++ to C#, not least because C# is about 100% slower.
I do highly doubt they are rewriting the engine into C# though.