oh lets not even start this. be careful who you call a moron, what goes around comes around.
If you are planning on running 10 psi on a stock GSR you will need a turbo that can push more or less 500 cf/m at 10 psi. If you get a turbo that can push 1000 cf/m at 10 psi you are simply lagging yourself up.
Now, if you take that same GSR and bore it, stroke it, and raise the rpms. You will need a larger turbo, and a larger turbo will do well on that gsr, whereas it would provide no benefit on the stock GSR.
The only need to get a larger turbo if:
1. you increase the cubic feet per minute your engine can displace...by either raising rpms, boring, stroking, or whatever.
2. if you plan to run more pressure at the same cubic feet per minute.
The more powerful your engine is to start with, the larger the turbo you will need. Not the other way around. A larger turbo will not make your engine more powerful than a small one, if they are running at the exact same pressure (to the map sensor).
This seems to be seriously mis understood, because in cases where you have a turbo that is too small for an engine, you are not actually running the amount of boost that your waste gate is set for. Even though your wastegate is set for 10 psi, with an undersized turbo at your redline, you might only be making 3 psi. If you left that setup the same, and put a larger turbo and changed nothing else. Your torque would increase from 20 percent over stock, to just shy of 70 percent over stock. but that is as high as it would go, as long as your waste gate was set to 10 psi.
If I am such a moron, go look up all those dynos, and post the results of the torque before the turbo and after.