I replaced my rears with 4" rounds. The problem that I ran into was that the cast basket for my new speakers was ~1/8" thick vs. the stock spekaer which was a piece of thin sheet metal. When I mounted my replacement to the plastic weather shield and then re-inserted, I didn't have the clearence to put the door panel back on.
I ended up building a mounting bracket that comes in from the back of the door and it works now (didn't use suby's plastic weather shield). If you go with a speaker that has a sheet metal basket then you shouldn't have any problems just using suby's weather shield.
My stock speakers that I took out were 4" rounds, but I didn't opt for the upgraded stereo from subaru. I also found the fronts to be 5.25's, which was a bit of a disappointment. Does the upgraded stereo system get you bigger speakers as well?