I'm still testing big loads with mono. Once I figure out all the performance bottlenecks, I'll write a blog post about it.
Mono gets quite fast if you flip some command-line switches. The defaults are a middle-ground between fast startup time and fast execution. If you're a webapp, you probably don't care about startup time (this is off the top of my head, i forgot where i saw the stats)
Can you give us some detail on what are those command-line switches?
You of course checked the latest version of Mono?
No, I haven't. I mentioned somewhere else that I know of the versions from 2-3 ago. I did _very_ extensive benchmarking, and they were quite far away in edge conditions (example: very high GC load).