Think of a golf ball- the dimples make it fly farther.
Also, fuel can collect on the port walls if they are smooth which is not a good thing. A slightly rough surface won't allow this. Polishing your exhaust ports though can help cut down on gunk a small amount, but probably not enough to warrant the time/cost.
I thought I read something on NECO's forums where the Leo Capaldi Speedvision Cougar gained around 40+hp with alot of attention to detain in the inlet side of the engine. Heads, manifolds, etc..
