I'm guessing an STi engine would require major cutting around the battery tray area to fit? The DOHC heads I believe are much bigger than the WRX SOHC heads.
Do you think going rotated with MAF delete and running speed density would free up any space? You could quite literally run a 6" or less intake/coupler between the turbo inlet and the air filter. All you would really need would be to plumb your purge valve to vacuum, and an AOS if you chose... Or do you think it will work better having the inlet under the intake manifold?
Actually, nevermind lol - I just looked at your first picture and it looks like you already have a turbo with the engine, so I'm assuming definitely not entertaining the rotated idea.