Does anyone have any suggestions for a bigger rotor/ 4 piston caliper configuration for the rear only?

I have already upgraded the front system and it just seems weird not to do the rear. Are there any direct MB replacements I can consider? I'd prefer not to buy something new due to the $ involved...

 

if you can wait a few weeks ill have a 4 piston setup that will be just what you are looking for. should be ready after the dragon