It's not really that simple. For instance, a given engine: a sized "small" turbo(compared to an oversized large turbo) that's running near maximum efficiency at 15psi would not only spool faster than a larger turbo at 10psi, but also make more peak torque and horsepower(higher static compression across the board).
To figure out what you'd need or is best, you have to do some calculations and plot some maps. It's the only way. And yes, PSI is relevant. You can't plot or map a turbo without knowing what pressure you want. There's two main things you need to plot your setup; the pressure you want and the flow needed at that pressure.
I have the same greddy turbo kit you arelooking at. I rebuilt my bottom end. Have dsm injectors. Some minor upgrades to the head. And im only running 12 untill summer. Pm me if you need help or have any questions I would be glad to throw you ideas.