to be honest its still not needed, the VAG group generally oversize their rear brakes out of the factory.
a massive majority of the braking force is done by the fronts, although the rears do help slightly with braking, they are mainly there for helping with stability on high speed braking, to give you an idea im running 4 pot racing brakes on mine and havent touched the rears as its not needed, and i also know a few guys with TTRS's running 8pot RS6 brakes and again they havent touched the rears and these guys track the cars very very hard!
another thing you would have to consider it if your upping front and back you may have to change the brake master cylinder to compensate or you could end up with a lot of pedal travel! so personally i wouldnt bother