Yeah I'm unsure if its the intercooler or the headers...I think any of these two would see a big difference, the pwest i/c holds more charged air and usually see cars making bigger numbers with that on...so does headers.

Mine is 8cm and normal 2.75 inlet size.

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06 STi: 337hp @23psi on E85...
06 STi: 309hp @22psi on 98ron...