I've heard that the Mediator is a pain to get to work with an 030, and you have to disable CPU caches to get it to work, which slows things up, not that RTG is very fast on an 030 at any rate.
I think you're better off sticking with AGA if you have an 030 CPU, Best not to expect too much from the classic amiga, it's getting old now in hardware and software

Robert