+1, I annihilated a diff with a 1515 1.5D on 4s so a 1521 is out of the question. I'd go with a 1515 1.5Y on 6s maximum (the E-Revo diffs are supposed to be tougher than the ones I had, hopefully that's true). The Tranny is pretty tough, but the bottom gear is plastic and a 1521 could very possibly destroy that gear.
All I ever wanted was an honest weeks pay for an honest days work.