Do you mind me asking why you feel the need to upgrade this caliper ??
Providing you don't clean out the standard rear caliper as often as forum member Darrsi does
the caliper is quite prone to piston seizure due to it's design. Water, salt & dirt can gather in it, affecting the seals and piston movement. There are some alternative rear brake calipers requiring minimal modification to fit, with the FZS1000 and
some XJR1300 calipers being favourites.
The design of these calipers allow moisture / road muck to drain out of them far easier. As a bonus they also have larger pistons, offering a decent degree of feel activating the back brake and the
perception of more precise braking. Although you can lock the back wheel with the standard rear caliper, the challenges in maintaining them through poor weather and the availability of FZS1000/XJR1300 rear calipers means that these replacement calipers are sought after.