Oh for crying out loud, not this nonsense again.
This depends... on a LOT. Ambient temp, barometric pressure, humidity, engine compression, condition of engine, what turbo, exhaust, intercooler type, placement, and volume, size of IC pipes, fuel delivery system, even temperature of the fuel, and another thousand other variables that I'm not going to go into.
This is why tuners are still required... they tune *your* car for the conditions you drive in. Anyone saying that 20psi is too much without all the rest of the information is talking out their ass.