If the car got droop screws, those will effectively stop traction roll, by limiting the the downtravel.
I cant recall the complete Dr. Diff starting setup for carpet, but I do remember this: HPI White and Yellow progressive springs (yellow up front) and 60 wt oil on all four shocks. Piston nr. 3.
There are several blue and green springs, so it's a little hard to guide. However, they got different length, so if you go to www.hpiracing.com
-> option parts -> spring chart, you should be able to identify them. If the current ones are close to the progressives I mentioned, then you probably should get some stiffer ones.
By the way, it's a good car.