There doesn't seem to be many reliable benchmarks available. I'm not sure why. Here is an in depth benchmark showing Mono lagging very heavily behind Microsoft, but I have no idea if the gap has widened or shrunk since then.
That article is 2.5 years old; Mono has gotten significantly better since Xamarin spun off from Novell, and much progress has been made in the last 18 months or so. It would be interesting to re-run those benchmarks with the latest version of Mono though.
AFAIK Microsoft's .NET runtime has also made significant improvements in the last 2.5 years, so as I said, it's just a thumb-suck as to whether the gap has changed. Maybe this will get someone interested in doing some comprehensive benchmarking...
In recent experiences, .NET runtime is faster in most situations, but sometimes Mono can be faster than .NET. It all depends on what you are doing. In extreme cases, .NET is usually ahead. You also have to account for the JIT time. It's after the JIT that should matter.
http://www.codeproject.com/Articles/212856/Head-to-head-benc...