'Official' word is that the 7900 brakes are not compatible with 7800 or older STI levers. Rear derailleur is cross compatible, front derailleur is not. I'd try the brakes but keep the 7800 front derailleur. BUT 7900 stuff is a lot more expensive than 7800 and probably doesn't work any better than 7800 stuff.
Thanks for the info. Now have to decide do I replace the levers or sell the brakes and front deraileur?? As got it all cheap when had to replace rear deraileur as 7800 spring isn't working properly. Cheers