Those 9" +50's would give you back 9mm more outside clearance. the 8" + 48 will give you 19mm more.

This will help u in your search:

Wheel Offset Calculator

the rears on the 05-07' STi's have a bit more clearance than a normal non-STi impreza body. but if you "roll" them the plastic overfender will want to pop off so some people have had to glue it back into place. I'd just run a lil more camber...

I personally like the wide low (Hellaflush) offsets. I'd go for the 9's so you aren't: