GT3540 is capable of making up to 30psi. Make sure your boost is low until you know exactly what kind of internals you have eg. big end bearings, main bearings, studs, forged pistons and rods. Valve train might be worth checking as well.
Running a turbo like that at, even at half its potential, can be dangerous on a RB25 with stock internals.
Exibit A:
18psi was running through this one when it blew, very nasty.
See if the previous owners have reciepts for these mods.
Good luck .