I go through this a lot with our inspectors. This is basically what I get paid to, but for radioactive materials and not air pollutants: interpret and administer the regulations. Same circus, different clowns. This took me about an hour to read, digest, and internalize.

The letter of the regulation is thus: CVC 27156. "No person shall operate or leave standing upon a highway a motor vehicle that is required to be equipped with a motor vehicle pollution control device." "No person shall disconnect, modify, or alter any such device."

A manufacturer can, however, apply for an exemption to that regulation to manufacture parts that either consolidate, replace, or remove pollution control devices if they can demonstrate that installation of those manufactured parts "will not decrease the effectiveness of required motor vehicle pollution control devices". These exemptions are issued as Executive Orders (EO) that exempt them from CVC 27156.

Currently, there are two intercooler kits that have ARB EO exemptions to be installed on WRXs/STis. The fact that the ARB issued EO exemptions to the regulations for ANY intercooler on a Subaru and/or Subaru motor indicates that intercoolers are considered pollution control devices or, at the very least, components of pollution control devices. Otherwise, intercoolers would not need to be exempted. One kit is the Easy Street Motorsports intercooler BUT only when installed as part of the Model ESX2005 kit (ARB EO D-588-1) which has a bunch of other stuff like fuel pump, etc. and part of the "ESX" Edition STi originally only for 2005 STis, but later amended to include 2004-2006 STis. The other is the Mishimoto performance intercooler kit (ARB EO D-759). These are the only two "CARB Legal" intercooler kits for Subarus. There are a few other kits, but they are for Subaru motors to be installed in VW Vanagons. There are also intercoolers as parts of supercharger kits for FA motors, but, again, only when installed as part of a whole kit.

Here's where the fun starts and the debate really fires up. The ARB issued GUIDANCE (operative word here, GUIDANCE and not REGULATION) that during smog inspections, the smog check technician "need not verify the EO number of aftermarket intercoolers installed on gasoline-powered vehicles equipped with OEM turbocharger/superchargers."

"So, I can run any intercooler I want! WOOOO!!"

No. No, you can't, because only the ESX and Mishimoto intercoolers are "CARB legal". The have EO exemptions and are good to go. For any other aftermarket intercooler installed on a WRX or STi undergoing inspection, the tech DOESN'T HAVE to verify the EO number, but that doesn't mean they WON'T or even CAN'T. Again, that was issued as a guidance and not a regulation. I’m speaking from first-hand professional experience: Guidance has ZERO regulatory weight. Ultimately, the regulation and the interpretation thereof by the administering regulatory authority will reign in a court of law.

Think of it this way:
CHP clocks you doing 66 mph in a posted 65 mph zone. Does the officer HAVE to pull you over and cite you? No. Are you breaking the law or doing something illegal? Yes. Depends on the work ethic and personality of the CHP officer if you get cited or not. Could the CHP officer use it as probable cause to pull you over for something else? Absolutely.

Same basic concept with some details swapped:
Smog tech sees an aftermarket intercooler (without an EO number) on your WRX. Does the smog tech have to verify its exemption from CVC 27156? No. Are you breaking the law or doing something illegal. Yes. Depends on the work ethic and personality of the smog tech to pass your visual inspection or not. Could the smog tech use it as reason to fail your visual inspection part of your smog inspection or to look for other infractions? Absolutely.

Long story short: Are front mount intercoolers on Subarus legal? No, but neither is exceeding the posted speed limit.