The tires you gain going with 18s over 19s is negligible. I just compared them and the only noteworthy tire you get, in tire rack anyhow, over the 19 is the
CONTINENTAL
EXTREMECONTACT DWS 06. All of the other tires are ****kickers and wouldn't ever be on my radar anyhow.
The fact is, ANY stock touareg tire size is very specific and difficult to shop for. You have to alter width or height in order to get the wild tire sizes to fit.
With that said, it depends on your idea of off roading. I take my 19s to the boat launch, down dirt roads, down rutted up service roads, through blizzards etc. Would I take 19s through mud holes, rock climbing etc? No.