ive just changed to 280mm setup, and to be honest im stunned at the difference. u wouldnt notice it with town driving. but see if your pushing it on a bit the difference is very noticeable, the car stops really well now from hefty speeds
as for the actual difference in comparison of the setups, although the surface area of the pad is the same, the actual gripping surface on the 256mm disk is spinning faster than the 280mm disk when travelling at the same speed - if that makes sense?