2 psi of boost is enough for more than a 10% gain in power nominally.

Sadly though these things will NOT produce that sort of boost whilst trying to flow 15lbs of air or whatever, ie they can produce that much boost when the engine isnt trying to consume it, but as soon as you open the throttle at high rpm its got no chance at all, and is just going to be a flow restriction in the inlet losing you power.

Dont tell him though, let him buy one and find out the hard way!