It won't increase your BHP, just makes your engine smoother by getting rid of flat spots etc..
There's been numerous threads about this in the past.. do a search.
If you want more BHP, get yourself a decent panel filter (Pipercross, Jetex etc) a decent performance exhaust (Jetex, Supersprint, Miltek etc) and a performance manifold..
Or go all out and fit a Supercharger or Turbo