For normal application seems to be fast enought.
Moreover this version is a complete stack of mono, is not a reduced version.

I already try to "port" (simply copy from my desktop) a winform testing application created using Visual Studio and works. The problem is:
- slow to startup the application
- is not integrated in the Hildon environment so you not see in the task list, and so on
- unfortunally some winform action caused the crash of the application because is not supported by hildon (example you cannot ask to the window to go maximized )

Regards the compatibility from N810 and N900 there is a specific assemby - maemo-sharp that is automatic generated by hildon include file that can be different between N810 and N900. If anyone not use N900 specific function I think that you can build the application for one device and run on the other.

Obviously the runtime need to be the right version, FREMANTLE use glib 2.30.3 while DIABLO use glib2.12.12 and so on ...