Note that I'm not necessarily asking these questions rhetorically -- I don't know a lot about this technology stack and want to validate my core assumptions.
So, yeah, there's a cost. Depends what the end user is using Mono for as to whether it's worth it or not I guess.
So, faster code but more memory usage and longer JIT time seems to be the reason why mono didn't just switch over to LLVM.
My bet is that they'd switch if LLVM supported the same features and architectures as their own, but just with better results.
Or... uhm.... game developers...
There is a partial patch on mono's mailing list archives that provides initial infrastructure for querying for those hints. The initial idea was to provide inlining hints for caller and callee.
If you feel inclined to have such thing, please drop by our irc channel or mailing lists to discuss this and contribute some code.
Who uses exceptions, anyway?