Which FMIC has the best gains? There is no irrefutable evidence that any FMIC has better gains than another. The consensus, if there is one, is they are all within 5HP or less, gain wise, of each other.

Perrin 02-07 WRX/STi FMIC Black Boost Tubes w/ Black Silicone

ETS 4″ Stainless Steel Front Mount Intercooler Complete Upgrade Kit for 2004-05 Subaru Impreza STI

I have a 2006/2007 model, any differences? Yes. There were slight engine bay changes in these years, so there are issues with fitment on these years depending on the manufacturer. Check with your vendor prior to purchasing to sort these issues out.

I have a 2008/2009 model, any differences? Yes. Your engine bay has an entirely new layout vs. the 2002-2007 models. Most of the models listed below are for the older models, so you will have to refer to the manufacturer/retailer for fitment.

Will a FMIC work with my OEM or JDM foglights? It depends on the year/model and your foglights. Refer to the manufacturer/retailer for fitment issues with foglights.

What is the best size FMIC to buy for my turbo? This topic is highly debated. The two general trains of thought are:
a. Bigger is better
b. A FMIC should be purchased based on need