The only benchmark I have done so far is Lucene Java,
binary distribution running in sunJRE 1.5.0_06 verses Microsoft.NET 2.0 J# in debug mode.
The difference in time to index the lucene-1.9.1/src/java tree
(about 200 files in 1.5 megabytes) was small enough
to be ignored.
And I'm sure that more detailed benchmarks would
give about the same results. The C# code was
generated (at least at first) by an automatic translator,
and it would be compiled into pretty much the same
common intermediate language as the J# compiler
would produce on the input Java. Then the serious
optimization work would be done by the same just-in-time
compiler back-end. Lastly the runtime I/O would be the same.
Considering the huge amount of labor that is involved
in maintaining a couple hundred source files with
about 35 thousand lines of code, I'm wondering what
the big payoff for that expenditure of human capital
is likely to be with the C# version.