you guys are arguing because you're talking about two different cars. The 08 has a craptastic cat back which is indeed pretty restrictive. The 02-07 on the other hand have 2.25" of fairly straight piping which is adequate for stock turbo setups. In other words, you're both right but you're both talking about different cars. However, the original poster has an 07 so confusing him about 08 stuff is not helping much twentyclowns.
To make it clear: 07 STi with a CBE is cobb STAGE 1!! and it'll work fine.
Oh, and I am the great recycler of info here maybe, but not a guru. Listen to Dumdum or nosenugget if you want some more technical stuff. I can and do, however, respond to the same questions over and over again more than they can stand to.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD