I'm using stock valving for the Koni's with my 500 front, 430 rear setup. I can promise you, without a doubt in my mind that with a Koni/Intrax setup my car would be a QUITE a bit slower through a Auto-X course. However, The Koni/Intrax setup is a great setup for a street car, not to mention if you are just starting out it can be much more forgiving and will have a less steep learning curve.
IIRC those spring rates will be close to the limit of what you'd want to do with stock Konis, but you're right that they should work. I had stock Konis with 500/350 springs in the Pumpkin for a long time before I did finally rework a lot of stuff.
The advantages of how stiff my suspension for the Auto-X course would not translate nearly as well on a road course, but I still think it would be a step above the other combo's currently being run. However for the type of driving I do these GC's have been EXCELLENT. I think 5+ top 20 finishes (in a field of 70-120 cars), a 2009 North Carolina Autocross championship win and still being undefeated in the C-Prepared class for the last 3 years proves that. Generally, the only cars running in front of me (That are not in my class) weigh below 2300 LBS or have A LOT more tire. And I assure you, It's not mad driving skill on my part, as the group I run with churns out Quite a few Nationals runner ups/winners or Regional champions every year.
IMHO the main advantage of stiff springs is that the car reacts more quickly to driver input. In competition that's pretty important when you're out for that last tenth or two. So I agree that ultimately a stiffer setup (to a point) will be better for competition, certainly over a streetable spring rate. Losing streetability is the compromise you take to do so, though.
Stiff springs will also limit roll, which helps reduce unhappy roll-related camber effects, which can be important on a strut suspension like the SHO has.
e.g., my SVT Focus was incredibly well balanced on the stock suspension. It didn't need
anything done to it as far as managing understeer/oversteer, etc. So when I prepped it for TTF and took it to Nationals I used all my classing points for tires (R6), thinking it was balanced enough that that would be a better place to use up classing points than a different suspension setup.
It was ok that way, and I ultimately got within about 0.2 sec of the PTF track record at PIR, but not very close to the TTF record that had been set in a Sentra Spec-V on NT-01s (i.e., less sticky tires) and coilovers. I think the Spec-V is overall a better car for that class, but even though the Focus was
very well balanced, it rolled too much on the stock springs (especially on sticky tires!) and the slow response to driver input didn't help at all.
So for serious competition I don't think there's a substitute for coilovers. If you're managing budget and/or want to keep the car streetable, then you do what you can given the desired constraints, and Konis and aftermarket springs aren't necessarily a bad way to go if that's the case.
