Some numbers I gathered during my fact finding mission . I couldn't find any spring rate for 997 cars. Note also that the GT3 is 300+ lbs lighter when comparing spring rates.

Bilstein PSS10 Damptronics
Front 285 Main, 115 Tender
Rear 570 Main, 145 Tender
More or less effective 285/570; tenders are fully compressed. I am impressed with the stiffness that Bilstein chose. Note how close the rates are to the 996 GT3's, admittedly a much lighter car.

Stock 996 TT:
Front: 187
Rear: 340

Stock 997 TT: No numbers available, but perhaps should be close to 996 TT numbers above?
Front: Conical shape means progressive?
Rear: Linear?

Stock 996 GT3:
Front: 225 Linear
Rear: 550 Progressive

Of the springs, GMG Springs are apparently stiffer than Techart, which is in turn stiffer than H&R. Note however, that some (or all?) of these springs are progressive, meaning that they are initially soft, then become stiffer as they compress. For obvious reason, none of the spring vendors reveal their products' parameters.