If you buy a turbo that become "effectient" at 15psi and try to run it at 4 to 7 psi, it will run like crap.
Buy a turbo that your current or intented mods will support.
Don't bother wasting money on a monsterous turbo that is slow to spool up and you will never be able to use it to it full potential.
Also look at the turbo's CFM rating. The amount of air it will flow at Xpsi. This will help alot. If you have a stock motor unported/without a full turbo back Ex, you may only be able to push 100-300CFM Max. No reason to strap on a huge GT that flows 800+ CFM.
Buy the right turbo the first time and not the BIGGEST you find at a cheap price. This will in the end give you the most bang for your buck.
Just my 2cents.
BTW I have a fully built "ground up" Balanced and Blue Printed 2.0L running a 800CFM @ 15psi Turbo producing just over 400WHP.
PS: One other thing, match the spool up to your driving style and application, Street / Street-Strip / Strip Only (etc.) You don't want a turbo that fully spools at 4500RPMs if you are looking to street race, light to light.