I thought it be more like 20hp. I know those 1970 hp cast manifolds can allow for some decent flow. I was running them on my 318 with 360 heads and 3.90 gears. Switched from cheap headers, notice a slight drop in power.
In there they did a dyno HP comparison with just the header replace.
Most performance parts are a compromise.... It will make more Power but loose torque, or have lots of toque/power down low but won't rev
THE HEADER IS THE EXCEPTION TO THE RULE!!!!
More bottom end power, more mid range cruise power and more top end power to boot(every thing in you power range is enhanced)
Go read that article and don't pass up the best performance you can add to a stock, or full race eng.