The Bilistein B6 HD high performance monotube shocks and struts are a great upgrade for a BRZ/FRS with either stock or aftermarket springs. Installing many of the available springs on the stock struts will result in an under-damped system and reduce overall performance and ride quality. These bilsteins offer more damping force to deal with higher spring rates and work well even with RCE's tarmac springs, which are one of the stiffest offerings available. The B6 and B8 in this application are nearly identical, with the B8 having slightly less travel to avoid some lowering springs unseating at full droop. However, they do not increase bump travel and do not change ride height like some versions of the B8, so for these cars it's recommended to stick with the B6 regardless of the springs used, for the additional travel. The front struts use 40mm inverted shafts to increase lateral stiffness and eliminate side loading on the damper internals.