I think -2 in the rear will be too much, I'll go for rear -1.5 if front caster is in 5-6 range. and then balance the car with tire pressure.
but depends mainly on the track configuration and driving habits. one setup works for one man on the track, other setup doesn't
I have -2.3f, -1.3 rear, 22mm rear swaybar, 6* caster. car is neutral on the middle speeds, up to 140km/h, after this tends to oversteer if rear tire pressure is the same as front. depending on the course I can adjust pressures and get the car feeling I want.