The 26 spline shafts are the stronger of the two. However, even the world class t-5's had issues with standing up to the 305's found in v8 camaro's. The only difference in the world class and regular T-5's is in the shifter feel. One will not handle more power than the other. They both have limits around 250ft/lbs. They don't do well for any sort of race application, but they are pretty easy to find and aren't too bad for motoring around on the street if you want to get decent mileage.
I don't have any information on performance rebuilds of any kind, but as common as that transmission is I doubt you'll have too much trouble finding any upgraded bearings and what not.