I am no expert on either car, but I would assume the focus would slaughter most little hot hatches out there right now based on the updates it just got. Or am I misinformed?
The ST? It got a brake upgrade in mid 2014, and the 15's got a revised suspension with stiffer dampers and a few other things. But power and weight remained the same.
In stock trim the ST beats the Golf GTI. But the GTI comes with a higher flowing turbo, and the DSG allows it to stay in boost between shifts. Its also lighter than the ST.
The RS and Golf R are about the same weight actually. And it has an additional 60hp and more torque out of the gate. But, the Golf R's DSG will provide big acceleration benefits by keeping the engine in boost.
The Focus RS doesn't actually have a center differential at all. The power is sent directly to the rear via a drive shaft, there is a clutch system in there that determines how much power is sent to the rear. It can send 70% to the rear, and in the rear it can send 100% to either wheel. The front remains an open differential using the brake biased diff system the ST has. For road racers a LSD for the front will probably be developed to increase front end grip under acceleration. And I imagine that there will be upgrades available for the RDU. Its a really light setup. The base RS weighs about the same as a base Focus ST.
I've done some flat foot shifting in the ST. It makes a huge difference. Its like a kick in the head every upshift compared to not doing FFS. But its pretty abusive to the transmission, so I don't do it. I have probably used the FFS feature about 3 times total in the years I've owned the car, I just don't feel comfortable with it.