Yep... Long Thread...
There is much discussion on this on some of the Impreza boards out there.
The design of the Subaru Intake track is very deliberate and designed to do a few things:
This seems like a normal thing for car makers to do. Keep it quiet, and make the customers happy. My NA Impreza and NA Outbacks aren't designed as aggressive performance machines, just great utility vehicles that need to sound a bit subdued.
-Tune the intake track for specific intake performance/economy:
As others have noted this isn't a deal on our cars, but Subaru does tune these to deliver a baseline for the MAF sensor and give a certain volume of air to the engine.
-Avoid resonance interference with the Mass Air Flow (MAF) sensor:
This is the biggy for our H4 NA engines with a MAF. The EJ25 engine will begin to cause resonance waves in intake track between 1200 and 2000 RPM. This is caused by the valves opening and closing, and the pressure differential they cause. At about 1800 RPM, the resonance gets very bad.
The Mass Air Flow sensor (MAF) senses air flow into the engine and helps the ECU come up with how much fuel to add to the air entering each cylinder. When the air in the intake resonates, the MAF can't correctly determine how much air is entering the engine. This causes the ECU to overestimate the air entering the engine and dump too much gas into the cylinder. This can cause a bit of a flat spot in performance.
The resonator that sits in the fender helps cancel out the waves in this RPM range. Without it, the engine can get a 'weak' spot around 1800 RPM, and over the long term the engine will figure out it should add less fuel. (long term fuel trim takes a big negative value). If the ECU didn't double check the MAF data with the upstream O2 sensor, there could be issues (stalling).
The resonance issue and it's relation to aftermarket intakes is best described here:
With the aftermarket Intake I have on my Impreza, I get to experience all the fun things mentioned above. Above 4000 RPM the intake gives my engine a bit more power, and makes a noise that sounds like my wagon is trying to kill something. I do see a flat spot in performance from about 1500 to 2000 RPM when under 75% to 100% throttle. What is very interesting about the aftermarket intake is what it does for throttle response. I used to think the Throttle by Wire on my '05 was causing the throttle response to be slow. It turned out the intake was a bit of the problem. With the new intake the throttle is more responsive.
IMHO... the Subaru intake is simply designed to let the EJ25 make a certain amount of power/torque, emit a lower level of sound, and do it without issue from idle to redline. Aftermarket intakes can help with a bit more power and louder acceleration, but they do so with a few drawbacks (low RPM flat spot, ECU having to correct)