Here are results of a simple run (average of three results, bigger is better) of tools from unixbench suite with WITNESS+INVARIANTS enabled and disabled:

This benchmark was performed on i386 Athlon64 1.6 GHz machine, on FreeBSD 6.1-RELEASE. There's no statistical analysis, but there's clearly an order-of-magnitude impact on some workloads.

WITNESS and INVARIANTS are debug options that should not be enabled on production systems.