Rear disks
 

Does anybody do any 2 piece rear disks for an 03sti?
Seen plenty for the front.
Cheers

Would be very hard to do if not impossible as the handbrake drum forms part of the "bell" on the back brakes. If you did manage to do one then the bell would need to be cast iron (or the handbrake would eat it) which means the weight saving would be small.

If it were not a road legal car, ie a race or track car, then if you were prepared to sacrifice the handbrake (or go hydraulic) then it could presumably be done fairly easily but I suspect the market would be tiny.