Very interesting thread
If I remember right, SPL says that going from OEM bucket to true type increases effective spring rate by 1.66.
Currently running a custom front splitter with air dam with around +.5 degree AOA with 3" of protrusion. Rear wing is a Aeromotions 72" (DO NOT BUY. It works well but a **** company.) with 6 degreesl AOA. Spring rates are 14k F rear 12k in OEM bucket.
So if I went to a true type I'd need somewhere around 7.2kg springs to maintain the current balance.
Balance of the car is setup to understeer slightly right now. with a degree less in the wing it was almost perfectly balanced.
I was running these spring rates without aero and now with. I definitely feel that I could use a bit more rate purely going off my amateur seat of pants feel.
Here's SPL's spring size calculator but it's where I grabbed the info about the spring location modifier (spring motion ratio)
http://www.splparts.com/content/SpringCalculator.xls