If it helps.. a good-power 300ZX that belongs to a mate of mine ran the other year at the pod... struggling for traction until we dropped the pressures from 30psi (245/40/18 or summat) down to 18/20 psi, didn't seem to help any less than that on his car, made a .7s difference, believe it or not.

Overcast day as well if that helps. Slightly damp track.