Porting will make a difference in both HP and Torque. BUT...
For the original 60 degree V6 there is a V6 performance book
(I forget by who, mine is packed away somewhere) that describes how
to port the heads.
Porting a 5.0 is very common. There are lots of shops that can do it manually and also a number of shops that will port the GT heads on a CNC machine. Expect to pay between $250 to $500 for the set.
I don't know anyone who has porting intructuions / data on the V6 SOHC engine. Without having some prior data / instructions your taking a chance.
I don't know anyone who has done a dyno test before and after on porting the V6 heads.
Polishing is not worth the cost unless you are looking for hundredths at the track. You will not notice a difference on the street.
Porting by itself won't do a whole lot, I wouldn't expect more than 5-10 HP. You also need to consider the rest of the system:
1) Larger MAS and throttle body, air cleaner
2) Taller , fenelic (sp), intake manifold gasket for a cooler charge and longer runners (more torque)
3) Different cam / valve setup
4) Headers
5) High flow exhaust (CAT, pipes, muffler)
Items 2 & 3 are harder to do, and may not be available for this engine.
Items 1, 4, and 5 - along with porting could result in something more like 20 or 25 HP gains, possible more.
Finally, you will want a re-programmed chip to go with your mods.