Think as a rule you can enough power out of the ST without the cost of pulling the head off it and having it ported/polished. No point polishing the inlet ports in any case as this isn't as efficient.
You can more or less take the ST to it's limits (liners!) by going down the usual route of remap, exhaust mods, CAIS etc and possibly a hybrid/big turbo so the extra time cost and effort in having the head modded just isn't viable for the gains you'd get.
All depends on how far you are going to take the car modification wise. Porting and polishing if done right can have great results as it not only helps get air in but also helps get air out. Personally i would say it does not need to be done on a car running a standard turbo as the head is def good enough for around 340-360bhp. The mian problem with the St block its the cylinder head temperatures hence now why a lot more aquamist systems are being used.
Collins performance are running a GT30 setup with a big valve head conversion, which is some serious wedge and are supposedly running around 380bhp on standard liners and pistons. This may well work but i would say is only a matter of time before it could all go wrong. I would rather have a reliable 300bhp then 380bhp for 5 minutes.
Advantages of good quality and measured port and polish are:
Matched inlet and exhaust ports, which allow for equalised air flow, pressure and volumetric efficiency.
Lower compression, thereby reducing temperatures in the head and the air at intake.
Wider exhaust ports allow more efficient scavenging of the exhaust gases.
Any work carried out though has to show proven gains on a flow bench as too much material removed will make the head worse. Also any polishing should be carried out to take into account the turbulence affect required to give a good air/fuel mix. Most heads are rough cast with the ports left untouched and this is where problems can arise and is one reason why some cars make better power than others.
When I had a P&P'd cylinder head installed to my supercharged ST170, the car made the same power but at lower overall pressure (down about 3psi). This is good as it means lower temps and thereby a more efficient engine.