Hi there,
As far as I'm aware there is no gear diff that will fit into the X6 - the SC14/B4.1 is too wide for the casing and I believe it is metric anyway. It's worth putting some ceramic balls into the normal diff, nothing wrong with the originals but ceramic make it smoother (I use vampire balls).
As for a base setup, check out Big E's setup sheets (
http://www.oople.com/forums/showthre...?t=548&page=16). At the moment I use red pistons all round with 37.5wt/black spring front and 35wt/pink spring back. Weight wise it's a matter of preference - I still run stick across the back, brass under lipo/speedo and 2 bars behind the servo. Recently though I have used a plastic front bulkhead but drilled a hole and added the extra weights that come with a brass bulkhead.
The car is very strong, front end is from a b4 - so wishbones/bulkhead/top plates are the normal things to go. The rear end is mainly Xfactory, the only things that tend to go is the odd hingepin/wishbone and wing mounts.
Kev