I used the Amsoil on my Forester, and I wasn't happy. For some reason car was using a lot of it, and talking with other people, confirm that. It not necessary mean Amsoil is a bad product, I just don't think it best choice for Subaru turbo engines.
I am still stick with old requirements and change oil and filter every ~3700-4000 miles, and taking into account you have 07 Impreza would stick with it.
I personally use Fuchs Titan Race Pro S 10W-50 plus STI pink filter. Perhaps I am a bit too much obsessed with engine lubrication, but prefer to be on a safe side. I personally think 5w30 might to thin, but that is my personal point of view. Was using 5w30 for a long time, and it was a bit like water.
On the other hands, as Don said I am putting a lots of miles on average ~600-700miles per week.