The link I tossed up isfrom Steve Timmins home page, he does loads of 3.6 swaps in 911's and he is a physics teacher at the University of Delaware. I think he is involved in the "Vehicle Dynamics& Powertrain Theory" program at UD, I met the guy once at Watking Glen and he seemed to know his stuff.
One glaring problem with the stock car product calculator is that no provision is made for torsional modulus of the steel. It appears an assumed torsional modulus has been incorporated into the applet. Porsche may use a different modulus steel in there torsion bars than was used in the applet.
Another thing to consideris do a pair of springs have the same spring rate?
I bought Hypercoil #550's for the rear of my 911, onespring was actually 553 the other 559... Its a small difference, and the difference vanishes when you corner balance the car.