People did this for years before the AP even came out. The factory tune is pig rich and I would argue the car runs better with a bellmouth downpipe than it did from the factory. You're basically letting the wastegate work properly rather than self-restricting the boost by basically shoving the wastegate flow down the turbo's throat. The ECU in the earlier years has a big open/closed loop break point (fixable by tuning) and basically when you get into open loop, it's just going to dump fuel in anyway so there really isn't much to worry about here more than a factory pipe. I'm usually the first one to scream "tune after every mod!" but in this case, it's not needed. Would it help? Hell yes. The open loop/closed loop transition alone is worth engine management to fix.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD