The increase in swept area associated with the larger rotors will aid in fade-resistance. The rear brakes typically do about 20% of the total braking, so fade in the rear probably isn't a big deal to begin with.
I imagine after adding an inch to the diameter and zero extra force (same caliper) you will have even fewer concerns with heat in th rear.
When Gary Sheenan had the stock brakes all around he was frying the front brakes, not the rears. Heat shouldn't be a major concern in the rear at all, especially with this setup.