The primary purpose of an aftermarket intercooler hose is to replace the stock intercooler hose with a better flowing unit. Airflow testing indicates 14.2% better flowing capacity than the OEM piece. It can also decrease intake temperatures, increase reliability, and dress up the engine compartment.

HP gain is 5HP. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" intercooler hoses on the market. Obviously, there may have been bad hoses sold, but not enough to report as "bad" overall. The current manufacturers of intercooler hoses that fit the OEM intercooler are:

a. APS: "Y" hose only
b. Perrin: 3 hose set
c. Prodrive: "Y" hose only
d. Samco: 3 hose set
e. Vishnu: 3 hose set
f. GP Moto: 3 hose set

What intercooler hose material is best? Intercooler hoses are made from silicone and cast alloy. There is no irrefutable evidence that one material is better than the other. Obviously, heat soak levels are higher with the cast alloy model.

Which intercooler hose construction method is best?
There is no irrefutable evidence that one design is better than the other. With regard to the number of silicone plys, here is a breakdown:

Perrin= 5 ply
Prodrive= unknown ply
Samco= 3 ply
Vishnu= 3 ply
GP Moto= unknown ply

Which intercooler hose set has the best gains? There is no irrefutable evidence that any intercooler hose set has better gains than another. The consensus, if there is one, is they are all within 1-2HP or less, gain wise, of each other.

Which particular hose gives the gains? The "Y" hose that attaches the turbo to the intercooler is the only hose that gives HP gains. The other two hoses, the throttle body hose and the recirculation hose, are nearly identical to the OEM size. Their purpose is mainly for under hood aesthetics.

Are there any downsides to intercooler hoses? There have not been significant amounts of problems with intercooler hoses. Many find the stock "pinch clamps" that are installed on the recirculation hose to be lacking, both in their functionality and ease of use. Many consider their replacement with standard "worm gear type" hose clamps to be a cheap and more user friendly substitute.

Where do I buy intercooler hoses? Every Subaru/Import performance store sells intercooler hoses. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install intercooler hoses? Allow around one hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.

How do I install intercooler hoses? Refer to the intercooler hose manufacturer's instructions. For intercooler hoses without instructions, below is a link to one of the better known intercooler hose installation instructions:
scoobymods.com instructions (with photos)

Should I reset my ECU after this mod? It is never a bad idea to perform an ECU reset after a mod. The traditional route is to disconnect the negative battery terminal, press the brake pedal for a few seconds to bleed the system of charge, and reattach. Some use the more advanced Vishnu Reset.