Heads are the key to making big power. If you get an aggressive head and cam package with oversized valves then you can turn your SC into a screamer. But you said, "port and polish" which is not a "big valve" head package. If you just tear your heads down and port/polish them then you won't notice as much of a difference as you would with a blower upgrade. Every little bit helps, and it depends on who is doing the porting/polishing, but I wouldn't bother doing anything to the stock heads and cam unless you are interested in making MAJOR HP.
It's already been shown that you can go with "bolt ons" only and achieve near 300RWHP and 400RWT.........which equates to anywhere from 375HP to 400HP and 500 to 530ft-lbs of torque at the engine. It would require all MP products along with a fresh air intake, MAF, injectors, fuel pump, underdrives, overdrive, ported manifolds, exhaust, whatever else and a good tune, but the results would be a relatively mild mannered SC with good idle. It would be relatively low revving which would make the power curves short and spikey, but the low revving nature would be better for longer engine life.
You could go with a head and cam package, but it would be $2500 to buy (the price of a MP2 blower and front mount.....which would be the same if not more power gain than the heads/cam) and you would have to put up with a lumpy idle which can be quite annoying if you have an automatic (occasionally stalling) and the increased power range would just put more stress on the bottom end. Not to mention the engine would probably be running a lot hotter.
That's just my opinion though.
Micah