I run on high bite clay with M4 Primes and here's what I've noticed.
2x1.6 Pistons make a big difference in handling and especially the 2x1.6/1.1 pistons from Avid or Schelle. Also I tried the AE springs and wasn't happy with them, I went back to using the Front 4dot and Rear 3dot springs and it makes the car feel more planted IMO and for my driving style.
Here's my setup sheet that I've tweeked over the last week or two since buying the 16 at the end of November.
Initial build setup from one on Petit RC database... fast lap 15.28 and average 16.08
Thornhill setup using AE springs (Blue & Black) and 2x1.6/1.1 pistons... best I could turn was 15.45 fast lap and average 15.9s
New Setup... switched back to Xray 4dot front and 3dot rear and dropped front shock oil from 42.5 to 40 and ran a 14.68 fast lap with average 15.2s