GPR used to do it but now only Eric Shea
http://www.pmbperformance.com does it now.
You can use the 914 rear calipers and have them spaced for vented 911 rear discs so that they match the front vented discs. The improvement when going to larger rears is minimal unless you are using a high hp 911 motor and track the car. A 914 stops with the front brakes....