After tuning a VF37 on a 2.5 should have at least 14psi of boost at around 2000-2500rpm. I am actually getting 16psi at 2500rpm right now with just an HKS EVC EZ II on my STi. However, boost will NOT hold more then 14psi to redline. Which means the internal wastegate actuator is doing its job. By installing a PE1825 or having a VF 36 or 37 rebuilt to be identical to a PE1825 (much cheaper route), you will be able to build boost slightly quicker then the PE1820, make more top-end then the 1820, and hold boost about the same as an 1820.
In my opinion one of the larger purposes of the twin scroll setup is not just spool-up. It's exhaust flow, which leads to greater throttle response......wait I take that back....rediculous throttle response