If you do the maths you will understand, but briefly due to the combined surface area of the AP's pistons being smaller than the standard 48mm pistons they do not exert as much pressure on the pads. In fact the AP's need a 280mm disc to have the same performance as the standard 240mm discs. The upgraded 280mm discs give about 6% more braking effort than the AP's without the cost and effort of refurbishing corroded AP's. I actually went one stage further and fitted 294mm discs on the front. With that increased performance at the front you really do need the 280mm conversion on the rear whereas with the AP's you don't. What is surprising is that MG fitted such small discs for a car designed in the 1990's, the rear calipers were designed to work with 260mm discs and cope with 280mm's with no problems, however the 280mm's with the standard calipers do lead to a small amount of pad overhang. I overcame that by using Mini R56 calipers that were actually used the 294mm discs by BMW themselves.

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