why not change the caliper too, youre gonna get uneven pressure on the pad cause the piston wont cover it all, plus the piston is no bigger so there wont be much increase in braking pressure and you might not be able to fit the larger pads into the caliper, why not use 2wd calipers too?

we upgraded to 4x4 calipers, disks, and pads on my mates 3i the other day, and all we needed was a couple of 3mm washers and it bolted straight on.

major difference in braking, and these were standard disks and pads!
before and after