I'm using the Contis on my AfricaAlp (modded Transalp). Running 28 front 33 rear. I've aired down in the dirt but it doesn't seem to have too much affect unless its real muddy.
The Contis have a fairly soft sidewall (you'll find that when you mount them) and are great on pavement too. If it's rocky where you ride, I'd start higher and air down slowly so you don't ding a rim. Im using Pirellis on my DRZ and the Alp is a bit heavier....hopefully the comments will help.