Lately I checked the Perlin noise performance test that was linked to on reddit (
http://www.reddit.com/r/programming/comments/31mzu1/go_vs_ru...).
Summary:
Benchmark: C# (Mono) - 1052.041381 ms, Java - 597.9874 ms
My machine: C# (Windows) - ~475 ms, Java - ~552 ms
Even considering that Mono is using maximum precision available (so instead of floats it's using doubles), when I swapped all floats to doubles it still ran in ~525 ms on my box.
So while Mono might be 'getting there', sometimes it's still lagging behind. It's just something to keep in mind :)