"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."

From automobilemag

coupe's & roadsters have slightly different spring rates also

"The Coupe’s spring rates are 451 lb./in. in the front and 480 lb./in. in the rear, while the Roadster’s are 434 lb./in. in the front and 491 lb./in. in the rear, compared to 303 lb./in. front and 337 lb./in. rear on the original Crossfire Coupe and Roadster models."

From topspeed

So N/A coupe and roadster has same springs, but on the srt they are different spring rates.
given that the roadster is heavier in the rear than a coupe with a empty hatch