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Originally Posted by MattR
No worries dude, I'm still learning too, if you guys saw my ride before my WRX you'd laugh, and understand why I know so little. 
Anyway, just a general rule of thumb with turbos, I've learned that aside from an Uppipe (pre-turbo cat), most of the horsepower and torque is made after the turbo. Exhaust, Downpipes, Headers, etc, Or with engine management systems. I haven't seen gains from intakes, or filters that really make it worth doing. I've heard over and over, to spend your money on these parts, and leave the intakes to the N/A cars.
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That's true, 'cept the headers are *before* the turbo.
In all car tuning, the way to make more power is to get more oxygen into the combustion reaction. On normally asperated cars, this usuall involved advancing the timing, or raising the compression with heads or something. On a turbo car, it's all about raising the boost. Unfortunately, turning up the boost isn't as simple as just tossing a Manual Boost Controller on there (a $12 part)... not unless you plan to bomb the motor in about 100 miles.
To make boost *safely* there's a few things that can be done. IF you can get the exhaust to flow freely, then you can spin the turbo up faster, and therefore make boost throughout more of the power band. More boost in the power band doesn't mean higher boost, just more rmps where the boost is available. That's the idea behind the up-pipe and turbo-back exhausts. However, on the WRX, it looks like headers don't really make much of a difference. The only thing really bad about the stock headers is that they weight a lot.
Another method of making boost, is of course the MBC, which allows the car to run max boost in every gear, or to run higher than stock boost. But to make it safe, you need some sort of ECU adjustment, so the motor can be supplied with enough gas to prevent the lean condiditon that leads to knock that leads to detonation that leads to a fragged engine. Vishnu does this with the Unichip piggyback computer. Basically, they can throw a cheap $12 MBC on the car safely, because there's a $500 computer making sure there's enough fuel in there to avoid detonation.
The reason intakes don't help a lot, is because on a turbo, you're already able to compress the air (increasing oxygen) far more than a free flowing intake will help. In fact the only gain from an intake on a turbo comes from the intake charge being colder. However the WRX already has a decent cold air setup (esp. if you left that snorkus thing on there). Since the Vishnu system is tuned for the stock intake, an aftermarket one can be dangerous, since it will increse the O2 to levels the computer isn't expecting, and you risk detonation. So, on a WRX the only reason for a CAI is for the awesome sound they make.