I don't see where you're getting the one fifth of high end gpu. Right now it would be about half and I don't see anything faster coming out til it launches. Unless you're comparing to Titan-SLI or something like that.
Plus, when the PS3 finally launched, 8800GTX was actually out (barely) and I would definitely qualify RSX as less than half as powerful. Even if you discount that chip, 7900GTX was readily available (though I actually guess RSX is about 3/4 of 7900gtx).
I am comparing it to a Titan or Amd 9970. My understanding is that both of these chips are at least around four or five times more powerful than a 7850. Also, the difference with the ps4 is that when it launches the titan will have been out for at least a year, and another generation of gpus will be arriving. By the time the ps4 launches, the gpu is going to be far less advanced than what the rsx was. For stylized or cartoonish games it will be great, but for pushing realism it wont be up to date.