Power isn't the only thing to consider. Things like lift and trail braking induced oversteer will be affected by increasing the size of the rear tires. It's very important to consider all of the aspects involved before changing things. Only time will tell what is fastest on the track.
It also depends on your driving style. If you want to go for a more traditional Subaru style, wider tires in the back could help you with the slow in, fast out tons of grip philosophy. If you want to go for something a bit more neutral, it may be better to stay square and sacrifice a little bit of your ability to really put it down out of a corner and increase your ability to rotate the car, allowing you to get on the power earlier and apply power more smoothly.
It's also possible I'm talking out of my ass here. I'm inclined to go with square setups if only for ease of tire rotation and longevity.
Would @
CSG David or @
CSG Mike like to chime in here? They have a good amount of track experience with this car already and may be able to give some input for those looking to go up in power.