Im having a similar problem at the moment mate. My car has a HKS GT-RS on it and the hks actuator is set on its stock boost which is ment to be around 12-14psi. Mine hit 22psi and was still climbing on the dyno today at 5500rpm, then the guys backed off. So i think my actuator must be hitting the split dump pipe aswell. However i thought it might creep up a little bit, but to 22+ psi?!?!?!?!
Mine isn't a just jap dump, but is very similar.
Let me know how you go, ill have to play around with my actuator this week sometime and check the dump pipe.