1. Mods have very little to do with the safe PSI limit. It is predominantly a function of the construction of the turbo itself. The factory turbo is built with a ceramic turbine wheel, and it is this component that is the "weakest link". As you increase boost, you increase the rpm at which the wheel spins. At a certain point, the ceramic unglues itself from the shaft and shatters. Best case is it sprays the front of any following car; worst case, it gives your engine's cylinders a terrific hone.
2. All you are doing is increasing MAXIMUM boost. It should still cut in at the same point, with the same "kick in the bum".