The Difference between a rear OEM caliper and underslung caliper / quite a few advantages to using this underslungnot (just the looks)
firstly the OEM caliper is a floating unit , and very grabby , so no real rear wheel control just basically an on/off switch with not much in between (most race bikes now have very little floating effect in the rear discs for this reason-fully floating discs have 2 braking actions-the initial contact/alignment and then the braking action-better to eliminate as much of that as possible with the rear-hence even the brembo front T-drive discs and galespeed etc don't have much or any lateral movement, and most race rear rotors are basically fixed
with the brembo fixed calipers you will be able to control the rear wheel/braking a lot more (thats was race rear calipers are designed for,not for braking force, but rear wheel control with lots of feel and modulation)
secondly rear wheel changes will be easier, and these set ups are much lighter than the std floating caliper with the big cast mount etc)
cooling is also improved, as well as lowered COG (maybe minimal, but everything helps) d one on a underslung bracket /