This thread has a lot of discussion about various spring rates and their effect.
m030 spring rates
Different drivers like a different feel so there is no one perfect setup. BSR Spec race cars run 450/500 and a lot of guys like it but I don't want a car so tied down. Too much spring on a car without LSD results in wheel spin on corner exit. Wheel spin is slow.
On my 986 BSX dual purpose Time Trial car I went with 270F/370R and it was *just enough* spring to reduce roll and make quicker transitions while still having full grip on corner exit. That car was good for 5 TT Championships so it worked pretty well and was still reasonably comfy on the street. YMMV