Whats the best way to get performance numbers for modifications to bitcoin ? Profiling it while running on testnet might work, but that would take a rather long time to get data. 
Is there anyway to speed this up  if we only needed to provide  relative performance between tests. (in a sense a fast performance regression test).

At least in theory, one possibility would be to replay real bitcoin transactions  to a test-net-in-a-box network (of maybe a few nodes)
that has the real blockchain history loaded into it but then operates at a far reduced hash difficulty in order facilitate running quickly.

Are there existing techniques/scripts for  some kind of perf testing with traffic that at least approximates real world traffic ? 
The test infrastructure  that ships with bitcoin does not appear to provide this.

Thanks,
Ian