My rig definitely stops better with the gm brakes after the swap, they have a wheel cylinder twice the size. you need way more pedal travel to do it but the difference is noticeable, and from speaking with some people that can be fixed with a mc bore change, as far as strength goes I cannot comment on longevity as I have not really pushed it but the 18 spline outers lived a long life on my willys with no trouble.
There is also a kit to convert to chromo 30 spline drive slugs and outers but from what light reading I have done on the subject it seems like the inner shaft or the ears on the shafts normally break before the 19 spline shaft does.
that being said from what i hear cal knows a lot about breaking stuff, he has probably forgotten more than i will ever learn