I wouldn't bother running anymore than 10psi. The stock cooler in the R34 is better, but still will have 2-3psi pressure drop, but increasing boost any further will add excessive fuel, and you'll find power going down due to overfuelling.
The R34 GT-t's internals are better, the compressor wheel is nylon(plastic) but the exhaust wheel is still ceramic, and won't cope with over-boosting too well.
I'm sure even with 10psi you'll make good power.