Have you tried using one of the newer versions of Mono (3.2.x)? The SGen GC is much better than the old Boehm-style GC, and SGen itself has improved by leaps and bounds over the past year.
Haven't tested on 3.2.x yet but the GC was never the issue for our stress tests. Bugs in the socket layers were killing us. Better under 2.10.9 but still not completely resolved.