[Shark_Bait88]

In respect to something like autocross, the FR-S and BRZ are fairly different cars in my opinion because of the difference in OE suspension.

With nearly twice the difference between front and rear rates, the FR-S is a much more tail-happy car than the BRZ. Subaru tuned their car to have a somewhat neutral feel, generally causing it to understeer/push a bit more than the FR-S. So they're really cars tuned for drivers with different style.

The rates on the TRD springs are essentially a stiffer version of the stock springs, in terms of the difference between f/r rates. So in my mind adding TRD springs to a BRZ would just make it handle like an FR-S.

Is that something you really want? It may not actually make the car faster if your driving style isn't suited to that sort of tail-happy rear bias suspension tuning. If you were actually looking to give the cars "equal" upgrades it mean being able to put stiffer lowering springs on the BRZ that more closely mirror the suspension characteristics of the stock car, so something with closer to squared off rates.

It seems to me that in the matter of performance/competitive driving, where suspension setup plays a major role, they are actually fairly different cars.