I'm running Aaron's Turbosmart Kompact with the stiff spring at the moment, its quite snappy when backing off but feels good none the less. The stock one was working fine with the grub screw mod but it could be due to the tune.
I ran 19psi on the stock turbo for 4 or 5 months, don't do it unless you are prepared to replace the turbo obviously, although I have a spare here.
To run high boost on the stock turbo, or any highflow version of it, you will need one of my modified actuators. That should hold around 15psi and allow over 22psi with a controller. Without the stiffer actuator it wont hold more than 14psi in the top end.