I can't say when GM made a large jump in exhaust manifold technology but if your running and older GM truck, I say about 85 and older your going to get a gain with a shorty header. GM had some older Rams Horn manifolds that worked very well, but they will not work with every car or truck.
I post this to not dispute the idea that shorty headers are waste for later model vehicles....the new GM and other makes flow very well and I would agree that they will be of no benefit for replacing these other than some sound improvement or they might be a cheaper replacement for damaged original parts.
For older vehicles they could be nice upgrade but you also have to watch your smog laws.