The one nice thing about the cen ring and pinion is your shim job doesn't have to be perfect. You can get away with a little play vs the XL stockers. Any play left in the stock setup and there goes another 13 dollars for a 13t pinion. I finaly threw away all the broken ones I had lined up like dead solders. I've been running the cen ring and pinion on the XL alum cup and gears.
http://www.rc-monster.com/forum/showthread.php?t=21219
The only problem so far was my screws backing out of the cup and breaking off. I need to get a new cup, I've been using a plastic diff cup from one of my rcs so I could get my Savage fix. A little more time to let the loc-tite set probably would have helped. Sound like the new HPI cup are a reasonable price and nothing to mod either.