The gearbox is not made for ball diffs - not seen what rclazy have done but unless it involves machining an alloy gear case it won't last
At the 2011 Worlds they tried to use HPI but they just stripped - because there's too much 'give' in the rear gearbox for superstar racers at least.
I have all the stuff to put ball diffs in my 410 (hpi) but never been bothered. I have a oneway diff, that makes me very happy.
I should say I think they have - or are - in the process of stiffening the gearbox for ball diffs.