The lighter bars have to twist more to support the same amount of weight. In the world of materials science, this greater twist for a given load results in a high level of what is termed 'strain' in the material. Higher strain levels fatigues parts more and distorts them more. So, there is some truth to the lighter bars being more prone to sagging and breaking. BUT, on the other hand a T-bar is a spring like any other spring. (In fact, a coil spring is just a t-bar in a sprial shape....) Strain exists in all springs: broken leafs and coils are not at all unusual.
No, the Mopar suspension is not substandard. In fact F1 cars went to T-bars some years back! Don't sweat that part.
The selection of spring rate in any car depends on weight AND application. The use of the 1.12" diameter T-bar is for a specific handling use. The application depends on what you want. I live in the rural Appalachians, and rally is my motorsport 'fix'. I would not have any use for the 1.12" T-bar on the local roads here (very bumpy in spots, and lots of sudden camber changes that require a lot of suspension travel to maintain traction). Ditto for rally use.
So you need to look at your roads and your intended use to decide. BTW, the standards of 'good' handling have increased somewhat from 1971!
