If you have something to boost the signal, an OD pedal or boost pedal for instance, you could do metal easily with the Mules, too. Last time when I had rehearsal with my band, I used a V-Stack pedal as boost, and my Mule-loaded Artist sounded heavy as hell yet really articulate.
HJM's right, the Mules are low-output, great-sounding pickups, and they don't help you at playing (= they show all your mistakes), which is a very good thing IMO.