The JRS springs that Mancini sells as USA made SS springs sagged about a year or so after they were installed. They worked okay but not optimum and as you'll probably already know, weight was not the culprit.
Like 383 said, they are okay for a mild to warmer/street/strip combo. Keep in mind that these newer springs may work better on some cars more than most and that is factored with many variables like convertor, driver, tires, track conditions, tuning, etc etc.....
The old original USA and Canada made SS springs were the darlings of yore, but only a true few NOS specimens are still around in peoples basements and are the likes of a treasure hunt to find.
I know of a 002-003 set of springs still in possession of someone here in Brooklyn since the 70's, but he does not want to sell 'em even though he is not racing anymore.

I personally love the old stealth look and to a certain extent, the old performance of the old original SS springs, but to try and catch up with today's technology, the only viable set ups that I have seen that work are the Tri-city launchers. Surprisingly, they are maybe a pound or two in weight over the Mancini SS springs and like Monte mentioned, it's all about the main spring, the thickness and the spring rate at front compared to the rear.
I have recently SPRUNG for a pair (They are up to or just under the cost of a Caltrac system) and we'll see how they fair after more TNT. So far, they are potent.
I bet some people have made their own and come off with good numbers.