Yeah, generally you want the rebound to be faster than compression off road. Keeps the car reacting to the terrain and keeps it from "packing up" and staying too far into the travel. On road I think slower rebound is what you want w/ the smooth surfaces.
I actually have a set of the Traxxas stuff, super simple shim that covers 2 ports on compression, but allows all four to function on rebound. Or the opposite depending on how you put them in.
traxxas variable piston kit
Couldn't go wrong for $6, I put them in my Revo and it didn't really work that well at first. I opened up the rebound ports to 1.2mm (they were less than 1mm) and it works great! I have 40 wt in there now and need to bump it up to 50 wt, it's a little soft. Compression damping is noticeably slower than rebound. I am actually impressed w/ a $6 Traxxas part!
I just got a set of Fioroni dual pistons for the D8T. Speaking as a machinist they are nice looking parts, I can't wait to play w/ them!