I upgraded the brakes on my car to the S5 spec. Bought a complete front and rear set of calipers, discs and caliper mounting brackets for £300 off Fleabay.
The front and rear calipers on the 3.0 B8 are the same as the S4 / S5 except the calipers on the S4 /S5 are painted black from the factory.
It's just the caliper mounting brackets that allow for the larger discs. They only fit the large brake dust shields on the B8 so it's a straight swap and the difference is noticeable. Larger brake discs allow better cooling and they aren't even expensive.
The front discs of the set up I bought weren't that good so bought some new ones from car parts for less. Pagid make for £28.45 each.
The rears were just fine so now car sports 330 *22 mm vented rear discs instead of 300 * 12mm solid discs.
Fronts on the S4 /S5 are 345mm * 30 as opposed to the 320mm * 30 on the 3.0 TDI.
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I fitted Mintex pads on the front but I think I might get some red stuff pads as the standard pads are quite dusty
Theres some pictures of the brake swap on my build thread linked at the the bottom. The larger S4 / S5 brakes fit under 17 inch wheels as well.
Karl