It looks like GM followed the footsteps of a few other turbo setups (older eclipse's, VW, and the SRT-4 ring a bell). They choose a small turbo to have instant feedback from the turbo, but because of the size it will poop out up top.
Most of the other markets have ported the turbos and increased their effeciencies, upgraded the compressor wheel and clipped the turbine wheel, or just upgraded the whole turbo in order to make more power. Each have their draw, and disadvantages too.
Depending on the car's ECU, portwork won't always yield you gains in peak HP. Take the SRT-4 for example, their PCM's are setup to calculate engine OUTPUT. So your dyno before AND after are the same after portwork, but at 2psi less (because of the increased effieciency) and your spool is improved. I have a feeling that the Sky's ECU will be similar. Now you can typically upgrade your wastegate or add a boost controller to get your boost back to where it was prior to portwork and net a ~20hp gain. Again, portwork is a good thing, but sometimes there are 'tricks' needed to see the benifits at the wheels. Usually this kind of work is relatively cheap, maybe $150-300.
The upgraded compressor wheel/clipped turbine combo will allow the turbo to act as if it were a larger turbo, increased lag a little as a trade off for higher peak HP. The clipping relieves the added exhaust pressures the larger compressor wheel brings. Typically these solutions will have porting as an option to offset the increase in lag. This kind of work is usually more, running $300-800.
The upgraded turbo as a whole is of course not only self explanitory, but also (generally) has the most options, and is the most expensive running $1500-5000
I guess I'm rambling, but I think your interest in portwork is solid, but you need to see if your PCM will 'let' you see the gains.
$.02