I know that Damon uses them, so they can't be shite... it just doesn't seem right to take a pad that's this good, and then coat your rim in tar... Having a rough braking surface for increased friction makes sense, and would work with the pad's compound. Tar makes pads stick regardless of compound. So that's why I said 'no'.
It's probably worth noting that the idea to use tar comes from a while ago when there weren't really aftermarket pads for your magura. One time you had three choices, black ones, red koolstop ones and green frog ones. Now there's sick pads, you shouldn't really neeeed to use it.