Just about to do the same thing with Tyres. Assuming 265/65/R18 is max size. Deciding between BFG KO AT2s and General Grabbers (Although not sure tyres size is available.
Also, my 957 2008 Cayenne GTS has air suspension, offroad its really useful to have the suspension on the full high setting, but it auto lowers above 20kph(roughly), has anyone been able to disable this. Dashing between dunes, its easy to exceed that, and it puts awful load on the compressor to keep raising it. (I've had the compressor overheat).