Do you have any basis to state that Powerchips claims would be overly optimistic? Have you dealt with them before?
Here is their page on the Crossfire modification.
http://www.powerchipgroup.com/datasheets/2/Chr0009.pdf

To me Powerchips claims could be just as accurate as Renntech's claims. The chip from Powerchip will not be available until 1/1/04 though.