I run up to 20psi on my stock engine and its going fine.
Others have run even more of course but I wouldnt push it past 22psi as a high boost setting.
The R34 highflows are good to around 250rwkw give or take 10 or so kw depending on your engine, dyno, setup etc.
It wont be laggy and you should be able to push around 18psi or something through the highflow.
Once you get into these numbers you will need a aftermarket ECU + Z32 air flow meter.
I dont know how much your injectors flow either so you might need different ones?