According to the Honda Press Pack for the SP2 at launch Honda quote the following "The new front brake master cylinder features shorter-stroke operation for more linear and responsive braking control. The two comact 4 piston front calipers were also found to run cooler than expect in most applications, so the insualtor backing material was removed from their pads for a slight reduction in unsprung weight."
You are correct when you say the calipers are different between the SP1 & 2 the major difference being piston diameter as follows:
SP1 34mm & 32mm pistons
SP2 32mm & 30mm pistons
So in summary the SP2 has a smaller piston size and stroke on the master cylinder and subsequently smaller pistons on the calipers which all makes sense to improve the feel which was Honda's intention. But what would be very very interesting would be to try a SP1 master cylinder (same size bore as Brembo upgrade item) with twin braided front lines which would beneift from the increase in master cylinder piston diameter working on the standard SP2 calipers. Anyone ever tried this or got any opinion on it as it sounds like a worthwhile upgrade, plus has anyone ridden SP1 and SP2 back to back to compare the difference in braking ?