I've done the HKS Sport exhaust, the expensive K & N filter kit (FIPK or something like that), sway bars, shocks, 245ZR16 tires and NA>T.
Of course the best performance was the NA>T.
But for me, the best NA mod was the suspension upgrade (Tokico TEMS, F/R sway bars, and 245ZR16 tires). The NA...