For what it's worth, I have the JBA long tubes and O/R H-pipe. Those, combined with a cold air intake, pulleys, and a 91 tune gained me 47hp and 47ft lbs of torque over stock. I have no figures for the headers alone however.
sounds to me like most people are getting exhaust setups for sound the add HP is a bonus. If what everyone says is true then out of the 47HP 5-10 came from the JBA LT's, right? Those numbers sound about right.
My baseline dyno numbers were 261.5 hp & 275.3 tq. I think a lot of the advertised gains are subject to the other installed mods. If you add all the supposed gains for each of my mods individually, it would be more than 47. Keep in mind though, this is only a 91 tune. I do have a 93 tune coming and that will put the overall gains over 50 hp/tq. Would have been much easier and cheaper to gain that and much more with nitrous, but this is always there, and my mpg at a 55 mph cruise is now 32 mpg.