that incrase of 5 - 6 BHP is the increase of peak performance, what you need to look at is the fact it should give you more torque all through the rev range.
Before and after runs on the same rolling road got me an extra 8bhp at the wheels on a modded 1108. Sammi's right - it's the mid range that gains, much smoother, picks up cleaner, no flatspot at 3000 revs, more there when changing up and mashing it again.