Piston size is FAR more important than any crappy branding on a caliper. Rear calipers have tiny pistons and should not be used on the front unless you want a **** pedal and **** brakes.
996 rears have a 30mm and a 28mm piston, giving a total piston area of 1321mm^2
Stock S3 front calipers use a single 57mm piston, giving a piston area of 2550mm^2
That means on a same size disk, the 996 rears will give you nearly half the braking torque for a given line pressure (ie pedal input).
996 Carrera/Boxster Fronts have 40mm and 36mm pistons, gving a piston area of 2273mm^2, so again assuming the same size of disk, a stock set of calipers will produce 12% MORE torque for a given line pressure.
I see a lot of cars with "Bling" 4 pots on them, but the reality is that most of the time its a backwards step, because no-one actually sits down and does the maths before nailing the calipers on.
If the disk is staying the same, you need to match the piston area as best as possible to ensure you dont lose braking performance. If your making the disk bigger, you can sometimes get away with dropping a little bit of piston area, as the larger active radius brings the torque back up.