Well, the thing about advertised gains, I've noticed, is that it's always the gain if that's the only mod and everything else is stock. In other words, it wouldn't surprise me if one really could get a gain of 20hp from just the headers -- if that's the only thing they did. As I'm sure you're well aware, once you start doing multiple mods, diminishing gains comes into play. I did the headers after I had already done the CBE and HFCs and intake. If you look at my dyno from the intake, for example, I got a nice gain out of that, even though I did them after the CBE and HFCs (+14.7 whp). But I think by the time I got to the headers, everything in my intake/exhaust was already pretty well opened up, hence any additional gain from the headers was minimal. Here's a link to my journal thread, btw. You can use it as a jumping off point to all my other dyno charts along the way if you want.
Semtex's Monterey Blue 370Z