The whole history of short tube headers (from what I have been able to figure out) is they were an emissions era performance part. Late 80s and those trying to hop up there mustangs and camaros but needed to pass smog. In order to keep the stock cats they came up with shorty headers that would bolt in place of stock manifolds or factory headers but would bolt to the stock cats. This was the late 80s, it was something. Not much existed. You got headers, but could keep the cats, pass smog check, and drive it on the street. They are not really performance parts, that is the long tube. And they still sold a lot of those for 'racing' that happen to end up on the street in places that didn't have inspections. Shorties are also good for turbo builds. Not for the performance, but the keep the turbo closer to the engine.
I'm not a fan of shorties. They are a marketing compromise. In the hayday of them, the typical advertising consisted of being an easy afternoon bolt on you can do at home without any special tools. Then there was a HP number tossed in that came from questionable data. As my last dyno tuner stated, what do you want it to be? Play with some heat soak and other engine/air temps and he can make the HP number swing different directions.