for short bursts?
are you talking about? It maintains 16PSI just fine if properly tuned but only for a given RPM range (smaller on the 2.5L than the 2.0L WRX's). It has a maximum airflow (CFM) at that boost however and as you ask for more air, the boost starts to drop narturally. All turbos taper off if you exceed their maximum boost point's flowrate which is going to happen on any small sized turbo like the stock TD04...
It has nothing do with the length of time (short bursts!?) and everything to do with the airflow. When the boost is tapering off, it's because your engine is pulling in more air than the turbo can spin up to full boost. It's a curve on any turbo: pressure and airflow. REMEMBER! Boost doesn't make power. AIRFLOW makes power (more specifically the combustion from it). Pressure is created because the turbo is trying to PUSH air in. A big turbo pushes more air at less boost than a small turbo. Boost itself does not tell you squat about the car and it's not "a bad thing" that boost tapers. People obsess about boost like it measures horsepower and that's pretty far from a direct relationship.
That's fairly off topic though. The issue here is how to effectively control boost which on older cars with less sophisticated ECU's, it can be done manually. On a WRX, the stock control is done exclusively through the ECU and is tied in with a variety of adjustments and is not so easily overridden. Rather than using a manual controller, the best way is to re-tune the ECU. The WRX's ECU is very programmable if you have the right equipment and can be set up for increase boost very easily (and adjusted for fuel as well) without butchering it's control over everything else engine related like a boost controller would do.
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