C# is pretty neat for game development imho.
It allows you to create complex structures without loosing track of the project structure.

MonoGame/XNA is basically a DirectX 9 wrapper (altough MonoGame uses GL under Linux).
I don't have a good experience with level design as MonoGame has no built in level editor (it doesn't even know the concept of a "level").
You must build everything yourself, even more than with Gamestudio. Yeah it takes some dirty work (loading textures, input, windows, ...) but it is not more than some kind of framework for writing engines and/or games.

You should take a look at some example projects to see how to work with it, i think this will explain best how MonoGame works.

Performance should not be a problem as mono and the .NET framework have a pretty good JIT compiler and you mostly don't do "heavy" stuff

Regards
Felix

PS.: If you ask for something specific, i may can answer the question.