Ian,
You know, I noticed that when we watched him race last year
at MidOhio.
(Close your eyes while I crawl way out on a limb here...)
Non-racer blather follows:
Have you weighed the car in race trim? I know it's much lighter
than stock, but if it's so light that the springs aren't loaded enough,
ride height (and therefore steering geometry?) may be compromised.
Kind of like a trailer misbehaves when it doesn't have enough weight
on it.
The rear springs might be correctly loaded only when the weight transfer
to the rear during acceleration is happening. It might be worthwhile
checking if the 50/50 weight distribution still the case. (A hydraulic
pressure gauge on a floor jack should be close enough... You
could even calibrate it in pounds, if you wish.)
I don't know what you're allowed to do in the class you run in,
but could you go to the earlier narrow rear springs (which I assume
would be rated for less weight)? Can you do the lowered spindles
on the front, in addition to 'weaker' springs? It seems that if the car
sat lower, the aerodynamics may be improved as well.
Keep in mind, of course, that the only race car I've ever driven
is on my son's PS2, so this advice is worth every cent you paid for it...
If you need a supply of 'pre-weakened' springs, you need only look
on the driver's side of our 40-year-old cars.
Ken in Columbus
Home of wild guesses.