Honestly I never have. Because in practise it is fast enough that it just didn't come up.

I'm sure that if I were to pimp the project properly then that would be a good thing to do, but half the fun of my proxy is to allow "interesting" rewrites, dynamically.

To benchmark I'd be too tempted try to game the results by doing a straight pass-through comparison to mod_proxy, nginx, etc. That would lose half of the appeal of the project in the first place.