The
JCW suspension lowers the car 10mm or 0.4" of an inch

. I haven't driven a MINI with the kit fitted, so can't comment what it's like. But there is a detailed review of it here
MotoringFile » Archive » MF Review: R56 JCW Sport Suspension MotoringFile » Archive » MF Review: JCW Suspension at the Track MotoringFile » Archive » JCW Suspension One Month Review 
.
The current
JCW brake kit isn't worth having. It just has drilled and slotted front discs, that are the same size as the standard discs. There just for looks really

. There is however a more aggressive
JCW brake kit coming sometime this year. It will have bigger rotors, better pads and 4 pot calipers. But it will require 17" wheels, with not all, if any of the current MINI designs fitting over them. So may well require some new wheels too.
Hope this helps.