Testing showed from 300bhp they allow mappers more head room, 300bhp became 320bhp due to to extra timing and such. Lag is a misconception as well, poor setup is the general cause. Only time when a front mount is pointless is on a TD04 turbo. After that its a much better solution.
It was tested back to back by jap performance mag (standard top mount, uprated top mount and front mount), plus the late great harvey smith did alot of testing and experimenting on the very subject. Problem comes down to poor airflow being cause by the scoop. Wind tunnel showed you loose a chuck of airflow into the scoop due to air ballooning.
I can't find the impreza image, but while the pic below is an svx, the areo is considerably better than an impreza and you can see you loose a section of air directly above the bonnet. Remove that from the area of the scoop and your reducign the airflow to the top mount considerably.
I get your point about trimming the bumpers though. personally i'd want the performance gain ;)