This is what developer Shootmymonkey (this post is from PSINext) had to say:
So overall RSX performance is basically a 7900.
Because of all the overclocked 7900 cards out there, I think few people know what the official spec'ed clock of the 7900 is -- RSX's is higher, btw. And the internal design specs are basically identical except for number of active ROPs. The 7900 has the greater fillrate, but RSX has the greater computational power. You can easily contrive a clear victory for either, but the practical differences won't be huge -- that's really what I was trying to say.
So overall RSX performance is basically a 7900.