This is a discussion of area related to the piston, not diameter. A is pi*r^2
A of 1.58 =1.96in^2. you have two. Double that.
So, 3.92 square inches per caliper.
A of a 2.75" single piston is 5.93"^2.
If you generate 100psi in the system, for example, you would have 593 pounds force on the rotor with the single 2.75 piston versus only 392 pounds force with the double.
Going to the larger 13" rotor over 11.75 nets you 10% better "leverage" but also increases inertia unless you got it to weigh the same. Remember, this is unsprung mass as well.
I missed that you had double piston calipers but my point remains, those rear brakes are huge and don't need to be, and just because they sell it doesn't mean you need to buy it. Your single piston calipers with the rear brakes you have gets you right near 66/33 distribution in force. Step down the rear pistons either thru diameter or metering or rotor reduction, and I bet you'll see an improvement.
