TurboWerx header claims vs reality

[Bobiam]

TurboWerx claims that their headers will provide up to a 25 HP gain, without other mods but a K&N filter.
For $800, even 15-20 HP, without loosing somewhere else on the power curve, would be money well spent.
Considering what some people have spent on total aftermarket exhaust mods it's hard to believe the Turbowerx claims. I haven't got the $ to get into anything worth over a grand.
Does anybody own this setup and have opinions to offer??????

Thanks, Bob