If only results matters to you, it should be in your interest to support faster toolkit which QT is and which is able to give you better results then Mono.
Since I work on graphics programming, I use xlib/OpenGL directly through C#. If the profiler shows a performance issue, I fix it. If I can't fix it through C#, I drop down to native code. Much more efficient and much less error prone than coding everything in C++.
I recently started experimenting with OpenCL, which provides yet another optimization avenue for code that needs it (highly parallel numeric calculations).
Originally posted by Apopas