Subaru requires no greaterthan 1/4" difference in overall tire height … that’s 1/8" of tread wear. Any more than that and you WILL toast the clutch pack in the AWD system. Figure on $800-$1,200 to fix that little jewel. Even running with one tire underinflated can wreck the AWD.
The sysmptom is ‘torque bind’. Take you 'roo to a dry paved area and turn as tight as you can in both directions and in both forward and reverse. If the car feels like it’s grabbing (don’t worry, you’ll know it when you feel it), ot in extreme cases crow-hopping, you have damaged the AWD. In some cases a total tranny flush and fluid replacement can help for a while. Don’t go to any of the nationals or local tranny joints for this, take it to Subaru.
For more info visit the Ultimate Subaru Message Board and search ‘torque bind’.