The car uses a much more sophisticated method of boost control. Even putting on an MBC and setting it to stock power levels would be a problem. In addition to a much more simplistic (read: inferior) system, the worst thing is the ECU still thinks it has full control of the wastegate to regulate boost as it makes subtle changes in fuel and timing. Raising the boost with an MBC goes further down the path of moronic.
There is no reason to use an MBC on these cars. The ECU is a boost controller which can also reduce maximum boost in partial throttle conditions, warm-up conditions, problem conditions, etc.
Take. It. Off.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD