The D2's are excellent quality, with less unsprung weight than stock components. Definitely they are more oriented for track use, and not anything close to a stock ride.
Stock spring rates for a MKIII are 461 ft/lbs front, and 215 ft/lbs rear. The D2's are shipped with 671 ft/lbs (12 kg/mm) front, and 447 ft/lbs (8 kg/mm) rear. Even with the minimum shock setting, these spring rates are quite punishing for a street car, particularly the rears.
Eibach Pro's are a progressive spring, fronts are 343-628 ft/lbs, and rears are 126-257 ft/lbs. Coupled with Tokico or KYB shocks you can obtain an uprated stock ride, sufficiently stiff to provide much better control, ride and handling.
Looking at the various springs given here, one can see what is a reasonable spring rate for a street MKIII.
Running D2's on my MKIII, I found the rear spring rates too high for street use. The front spring rate was stiff enough to give the car that 'bouncy' characteristic, and not very pleasant on broken up road surfaces even with shocks adjusted to the softest setting possible.
Since I really like the D2's for quality, weight and adjustability, I'm going to swap the rear springs to the front struts, and order a softer spring for the rears, probably the 196 ft/lb (3.5 kg/mm) setting. This will give approximate 'stock' spring rates on the car, and allow me to use more shock adjustment to balance out the ride and handling to my liking for street use.
D2 has replacement springs in stock at their place in Durham NC, I believe at $50 each.
Once I complete the swap, I'll post my findings on the results.
John