Here is some info I found on spring rates.

The SRT-6 shares an awful lot in common with the outgoing SLK32 AMG, other than just the engine and gearbox. The dual control arm front and multilink rear suspension are virtually identical, although they are tuned in this application, with firmer damping and much stiffer spring rates. (They have gone up from 303 pounds per inch front and 337 pounds per inch rear to 451 lb/in and 480 lb/in respectively on the coupe.) The brakes, too, have come in for attention, with vented discs all around (the stock car has solid rear rotors), bigger front discs, and dual piston calipers in place of single-piston items. Chrysler Crossfire SRT-6 Preview & Specifications - Automobile Magazine

OR:

The SRT-6 package includes stiffer spring rates (50% in the front, 42% in the rear, with higher jounce and rebound rates) and larger brake rotors, with vented rear discs. Oddly, there is no manual transmission available - a reversal from, say, the Spirit R/T, where the high-po version only came with a stick. - From www.allpar.com: Chrysler Crossfire and SRT-6 general information and car review

Cheers,
KrzyFast