i wouldnt push more than 20-25 psi if its stock
if you put in head studs,it helps
i used to let my old 1.6td go to 40 psi with studs
its not really the boost that will "blow it up",its the amount of fuel
i used to go 40 psi with the stock 9mm inj. pump,but when i did it with the 12mm pump i installed later on ,i started popping head gaskets