Upgrading the rears helps to keep the car more balanced under braking, stops some of the forward weight transfer and suchlike, however go overboard and you'll lock the rears under heavy braking.
B5 S4 used 321mm fronts and 256mm vented rears, so going to 256mm rears with the 312mm fronts would probably be ok, however they wont fit a FWD A4 as the hub and disk are integrated into one on the FWD b5's. I've looked into using A6/Passat hubs which might work, but without getting the parts and trying them on the car its hard to tell. If your interested i can dig out the exact parts i think might work and you can see if you can get hold of them...
by non-S i meant S4/RS4, the Sport models have the same brakes as the SE models.
Theres another thread about vagparts closing up shop, so your likely out of luck there. I got mine my emailing everyone on ebay thats breaking a TT for a few weeks and finally managed to procure a set. Some of the larger breakers on there tend to sell the carriers and calipers complete as an upgrade "kit" for Golfs etc, and as such want big money for them, but if you keep looking you'll find someone willing to sell just the carriers.