Rear differential bushings are much stiffer than the stock rubber bushings, every ounce of power will be directly transmitted to the rear wheels instead of absorbing the energy into the soft rubber.
Note: On the diff body 6PCS/Set
Scion FR-S 1st ZN6 2012-2016
Subaru BRZ 1st ZC6 2012-Present
Toyota 86 FT86 / FRS-S ZN6 / ZC6 2012-Present