Coilover spring rates, 93 Ford Probe GT SCCA car

I have a track car I'm building for SCCA Track Days in America and other track events, and I don't know how stiff to make the suspension. I have an AWR coilover set made specifically for our cars and it comes with 475lb Hyperco springs that are a bit short. I thought about adding helper springs but after I pay for springs and perches I could just buy a set of actual racing springs that fit. I'm worried the lowered spring height will bottom out the Bilstein inserts and blow the shock absorbers.

I want it to be stiff enough to minimize roll or eliminate it. This is not a daily driver but it is road legal and I'm only lowering it maybe two inches max, they already ride low from the factory. Will 600lb springs be too much? The car weighs 2850lb factory and it will be under 2500 after weight reduction. It has the big V6 in the front, big brake kit, 7/8" swaybar in the rear.

I'm not bound by any class restrictions (yet) my plan was to build it to the moon with no restrictions and then figure out later where I can't go.

Our ZX2 autocross/track day (Road America and Milwaukee Mile infield) car runs 750/650 f/r.  I think the suspension is design-wise similar on the Probe, though the ZX2 is closer 2300 pounds.  You can see in the photo that there is still a bit of lean in an autocross setting. 

I think the 600# springs are a good starting point for you.  You may end up going stiffer but a track car and one built for auto-X tend to be set up differently.

You want some give in the suspension on track. Hitting a bump in the middle of a turn when your at or near 100% could mean some scary moments at best or at worst looking out the pass. side window to see what you're going to hit.

Yes that was my big fear was being bottomed out and hitting something, the longer spring will give me more travel and some breathing room in places like the Gravity Cavity on the Dragon.

I think I might do 900's up front and 700's in the rear as a good starting point, I can always change them out later, the Probe is heavier than that ZX2 but they do share suspension components, a lot actually. And the Probe has a LOT more weight over that front end than the ZX2 and thats before I put a turbocharger on it.

For the record, they share engine mounts too, so you can KLZE swap them and have the glorious howl of that 7500 RPM Mazda V6.

They also have 525s and the setup I have uses the same length springs all around so I can just get a pair of 900s, 700s and a pair of 525s and go from there.

FWIW, there are online calculators that can get you close to what you want/need. You just need to provide the input (measurements etc) which IMO is better than guessing.