Since oem rear calipers are being used on the new 14" rear rotor's... they must have used some kind of offset spacer ? IE: the oem 1 piston rear caliper still has the same surface area... but is grabbing the outer portion of the 14" rotor. IOW, it's still not grabbing the un-used, inner portion of the new 14" rear rotor.
Ok, are you sure the 4 piston front calipers will even fit in the rears ??? And clear the 20" rims ?
On a similar note, if I have it correct, the rotor's are just big heatsinks. One way to cool things down a bit might be to air cool both the front + rear rotors. Air is free, so why not use it. I put the cdc brake ducts + 14" front rotor dust shields on my 2010. However, I'm still using the oem 12.4" front rotor's. My front 18" x 9" chrome bullitt rims won't clear the typ 14" rotor + 4/6 piston BBK.
The CDC brake duct kit uses 2.5" silicone hoses. I believe the FRPP kit uses 3" silicone hoses. Adding hoses to the rears would work, but might be a bit of a trick to install, and clear everything. 2.5" to 3.0" hoses..morphed down to an oval shape might be the ticket. At least with any rear hoses, they are fixed..... not like the fronts..that have to be long enough to allow front wheels to lock side to side.
Jimbo