Actually, respected tuners such as COBB, recommend the stock RAM air intake over most aftermarket ones out there. Remember that the stock intake for the WRX/STi already pulls cold air from the outside.
The problem with most aftermarket CAIs, and SRI's, is:
1) When installed, the MAF sensor is in a different orientation from stock
2) Different piping bends prior to the MAF result in a different "air flow" pattern (be it turbulence etc.) over the MAF, as opposed to stock.
Either of these can result in the MAF getting an erroneous reading, can can result in a lean out condition.
Your best bet IMHO would be to stick with the stock Ram Air intake. If you want to get more power out of it, look at removing the "resonator" on the stock intake, resulting in a free-er flow, and louder turbo spool. Also, be sure to use a drop in free-flow air filter, from either Perrin or K&N. Stay away from the HKS or Blitz Panel filters as the foam in these tends to disintegrate w/ time.
Here's a link:
http://www.fastwrx.com/insnre.html
