Originally Posted by
timmyquest
My 7800 stuff shifts pretty good

Mine too.
Friends do not fear about the incompatibility at least for the shifters. I heard from my LBS that the shifters and FD of the 7900 are compatible with 7800. Its just that shimano does not want us to buy piecemeal for a complete 7900 gruppo is "the best shifting experience" cough* bullcrap* cough...