Like Kimyee said, its gonna vary a great deal depending on sooooo many different things.
It(at least a 1g) will be the limiting factor for sure. Especially the downpipe. Its quite restrictive. I've seen cars with 3" turbo back, mild boost upgrades and an open filter outperform cars with much bigger turbos, racegas, better clutch and more on different occasions. The stock downpipe is one of the weakest parts on these cars.
an engine is an air pump. the more air that it flows thru it, the more power it makes, as long as its all working correctly. You can get as much air into the engine as you want, best engine, great cams, plenty of fuel, great manifolds, $$$ turbo, but if it can't get out, its not going to make any power hardly at all.
Think of your house or apartment at the end of a hot, hot day. Cooler outside but stifling inside. You open a window on one side of the house. Its a bit cooler near the window, but not that much better. OK, so you close that and open a door. Well, its better, but still only near the door. "Aha" you think, and head over to the other side of the house and open another window or door. NOW! you can feel some cold air flowing thru. Its cooler in mere min because the whole system is now flowing.
