The only are where the S1 and S2 boxes are stronger than teh rest, is that they use slightly larger gear teeth in the crownwheel and its pinion. Othervise they are teh same as the rest from apart the lsd of course.
Diesel boxes are stronger only if one means the mk6 escort diesel IB5 box. Same goes for the escort 1.8 zetec box (not the 130ps version with MTX75). These have a couple of bigger bearings and different syncro desing (double syncronised) for the first gears.