Got to get the OP to change the boost to 26+ PSI. My applause for posting the real boost in bold. 20 PSI to achieve 600WHP is not mathematically possible with the displacement of this motor, turbo size, and airflow. How do you like the AEM V1 on the Subies? To be honest, I have not liked them at all. The AVCS(not needed here) control is not very good. Have you tried the VIPEC at all? I have heard good things there.
The OP stated he wanted to change the wastegate to hold boost better. I think he needs a different gate to control boost better. Sounds like it is creeping due to gate size or design.
Do you think is it the size of the gate or is the design favoring the turbo? Sometimes the gate positioning can lead to creep. If you are able to split the flow as equally as possible on the up-pipe you can get a little better response from the smaller gates.
Is this going to be a drag only car or is it going to see some street driving?
Keep up the good work, and enjoy the car. I hope the OP has some experience with high HP cars that come in late. This will be a bit to handle for an inexperienced driver. It will def. light up the tires in the early gears.