DCR59 : Your post is very interesting. From the work I have done I see no point in going with cone filters until somewhere over 350 bhp. Apart from anything else, you loose the advantage of cold air, good filtration (Subject to air filter used) and the snorkle. Removing the snorkle on its own, contrary to Scooby myth is a BAD thing.
If you are going for a cone filter then you must fit a cold air divider to derive reasonable benefit.
Do NOT use aluminium. Aluminium is a good conductor of heat.
I use carbon fibre with a layer of heat mat on the engine side and I have a cardboard template here for the basic shape. This buds up to the bonnet and can be finished off with a rubber seal of the same type you find on door seals for various cars or from "Frosts"
I am happy to send you photos of the template, cut carbon fibre, heat mat and top seal if you wish to contact me by e-mail. There will also be photos of how to get lots of cold air in there. This is applicable to cold air divider and not O/E air box. If you know how to post photographs, feel free to link them on here.
Removing the rubber in front of the bonnet by the headlight is a good idea. More cold air goes over the top of the engine on an O/E set up but with an air box there is more cold air. (You know by how dirty it gets with muddy water marks).
The design intention from Subaru, in respect of air flow was that air came in at the front, through the radiator, over the engine and down the transmission tunnel. This flow was assisted by air through the bonnet scoop.
The grills on top of the bonnet are blanked off deliberately on an O/E car. The area on top of the bonnet is a low pressure area and removing the blanking plate(s) sucks air out. On a standard car this disrupts air flow and under bonnet temperatures will rise. 6-12C There are cases where removing the blanking plate or part of the plate is beneficial but without good test gear you cannot be sure of the results you achieve.
I have never used reversed scoops to force air in as I have doubts about the advantages over a well thought out flow system.
But this basically shows that an induction kit on it's own is a waste of time - unless you can get plenty of air to it and shield it from turbo and manifold heat. I know that most of you will know this already, but I wanted to see hard test figures to back this up.
This is true but as dynamix pointed out:
The thing to measure is the temperature of the air going into the engine really but I agree. The standard CAI kits are little more than Hot Air Induction kits and will kill performance.
Once you understand what is affecting inlet temperatures it is time to look at temperatures before the throttlebody but that is a topic on its own.