Ok, if anyone is looking for direct replacement speakers for their Impreza, a set of Vauxhall replacements are being sold on talkaudio, which is a highly reputable car hifi forum (they even have trader feedback - hint hint!)

I can't guarantee these will fit your Impreza, but many people use the Corsa adapters to fit 5.25" speakers, therefore these should fit.

Here is the sellers thread:

Talk Audio Forums

And here is some more info on the speakers:

TS-Q172C Pioneer Custom-Fit Speakers - Car Entertainment

I have no financial interest in this whatsoever, just looking to help people on both forums out.

If you are interested, might be best to PM me as you won't be able to post on talkaudio for sale forum unless you have 50 posts on there.