I'm running a 501x worlds with the 6.5 x12 and a sphere TC with no problems. As for the diffs, I've run both the front and the centre one way and I find the centre one way is the best. Its also the cheapest at £16+pp from Hongkong.
My worlds came with F+R roll bars so Ive kept them on but they don't seem to do much. If you want a set then
http://davesrcmodels.co.uk/ has them listed at £8.95 +pp which is cheap.
I run the none BRCA trackpower 3200 lipos at the mo which require only slight mods to make them fit. With lipos I dont add any weights (the car is still over the min weight) but the shocks need altering. I use tamiya springs, for indoor I use Medium at the front with 35wt oil and soft at the back with 25wt oil and set the rest of the car to Jimmys setup. I use a Db01/501x hybrid for outside but I'm still messing with the settings.
Hope this helps.