Unless you have the heads off for some reason like replacing head gaskets etc... I wouldn't go through the work and expense of porting and polishing heads on a basically stock engine, you will see some gains but it's a small amount unless you accompany a p&p job with mods such as higher compression pistons, bigger valves in the heads, hotter cam setup etc... and also if the person doing the porting doesn't really know what they are doing or ports them out too much for the cam and valves being used you may actually loose power do to the heads volumetric efficeincy being lowered.