Transnexus has done an interesting analysis of using RTPProxy 1.0 with OpenSER 1.3, of course specific with their use case.
It has very interesting results, you can check here:
There was a lot of discussing regarding space for improvements and tunings to get better performances on the mailing lists, but still it is a good reference to be used. The page includes details about everything you need to do the test by yourself.
Transnexus is the developer of OSP module in Kamailio/OpenSER.