[Casper]

Just did an interesting comparison out of curiosity.
I currently have both a set of BA front calipers designed for 298mm rotors and AU2 calipers designed for 287mm rotors in my shed.

Looking at them separately the BA ones appear to be considerably bigger than the AU2 ones, as you would expect given they fit a disk 11mm larger.
Put them side by side though and the phisical size difference of the calipers is basically nil.
Take the pads out of each and the same applies. The BA ones look bigger until you match them up and they are almost identical. The AU2 pads have a more pronounced curve as you would expect to fit a slightly smaller rotor. Overall pad surface area is effectively identical though.

So there you go, the braking difference/effectiveness between AU2/3 and BA is entirely in the disk size (as opposed to AU1 to AU2 where the disk diameter is identical and the calipers are significantly different. Disk thickness is different but not a major factor).

This sort of comparison backs up the packages you can now buy on Ebay that uses the calipers for the 298mm rotors and converts the mounting to allow them to be used on a 330mm rotor. Same pad surface area, bigger rotor, more brake "leverage".