Hey Tree, what caster do you run?
I assume you have the early adjustable caster rods, so if you can add a bit more caster, it may help with understeer. (maybe a nice cheap mod for ya)
Re springs:
Some minor details I found with Eibach Prokits.
Although similar, there appears to be slight differences in the ride heights between them and later ('92+) OEM springs.
Compared to stock '95 Bathurst springs, I found with Eibach Pro-Kits my car was higher (10mm) at the front and lower (8MM) at the rear. The result was I didn't think the heights matched F/R
The pro-Kits also rode harder. The comparison was made with Koni inserts on the softest setting.
The Prokits are also about 2kg per spring heavier than the stock springs.