You may run into some problems since not all 8800GTS's have the same exact chip or memory type.
You would be better off just overclocking the standard XFX to the XXX XFX speeds and see if its stable.
The reason the regular one didnt become an XXX, was most likely because it couldnt handle the clock speeds, so by flashing it, you risk having the clocks set too high, thereby not even allowing you to boot into windows without errors.