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Induction kits dont do anything for Imprezas (unless you have a monster); as suggested above Id look at an 'STI' filter element (not simply the filter element out of an STI btw).
I have a filter testing machine on loan from K&N filters. I tested the standard STi panel filter from my own 05 Sti and the results were very dissapointing. I have fitted a Carbon Viper kit (Pipercross) and I can notice a difference in performance.
If my engine was to go bang, I'd put the OE box back on as its a 10 minute job. The dealer would not know...
To back up what Codriver says, I have seen a couple of tests whereby the STI panel filter that everybody raves about came out worse than the stndard OE filter.
best part of £50 when £10 will do