The front end on my car is IRS. A really nice set up when I built it but now I think the CRC front end is better, but less adjustable. The IRS gets some slop in it when it wears a little.
When I built the car I wanted to keep all the aluminum blue so I used an IRS diff, the stock diff sucks because it only uses six balls. For performance the best diff is from Niftech.
The pivot balls form Niftech (for the front suspension and the t-bar) are heavy but fit tighter than others I've tried. The set-up tool from Niftech really works great and is quick although you can do the same with a good set of calipers (that being said I always use the Niftech tool).
From Niftech:
Diff, rings, right side hub (the left side hub will make your car too wide)
pivot balls
diff balls (if you use steel-I do not)
diff lube in the syringe
micro bearing oil

BTW-cut two slots in the chassis next to the t-bar so you can tape the cells in two on each side-much more secure.