Changing head?

Does anyone know what sort of performance gains i would achieve in terms of bhp with a port and
polished head? I have an induction kit, half system exhaust and am going to get a 4-2-1 manifold.
Are there any places that would do a discount for my cylinder head for a port and polished one???
Im looking for something quite noticeable so would it be worth doing or aim for an engine conversion?
Cheers

Depending on the condition of the bottom end somewhere between 15 - 20 bhp, with a fast road cam included maybe 30  :dontknow: