Okay, usually you can get away with this kind of mix and match even though Shimano says they are not compatible (not even B level compatible), and I'm usually the first to push the envelope but I have to say I wouldn't recommend this.
I recently bought a 2010 Specialized Tarmac Pro SL which came with mostly Dura Ace and Ultegra brakes, FD, cassette and chain. The ST-7900 only barely pulls enough cable on shifts to span the the big and small cogs on the FC-7900 39/53. Yes it works but you have to adjust it very carefully so that the plate isn't rubbing on the chain and if you shift up then pedal the chain wont jump, you have to be pushing the lever AND pedaling for the chain to move. Back pedaling has twice caused the chain the jump inwards and end up wrapped around the bottom bracket.
There is also no trim on either big or little ring, despite at least one Shimano document claiming there was trim on the little ring but not the big. Bottom line is I will be replacing with an FD-7900 at the first opportunity.