Whacky PRO3
Have you tried a closer spring rate spacing front to rear. Without the Upper deck you could run Pink Front/Blue Rear. I like my car with only 1 step difference in spring rates F to R. Without any other changes, installing the HPI Upperdeck will require softer springs (at least mine did) all around. You also might try -0- deg toe in the front. This should tame the car down a bit. If your class allows you to run a Front One-way this really helps in the corners.
Per recommendation of fellow Pro-3 owners on this Forum, I will be racing this weekend with the rear Roll Center (Camber Link) in the upper hole on the rear up-rights. This is susposed to help Mid-Corner steering.
Good luck racing,