I use - willtheyfit.com to figure out how a wheel would look and what issues I will have.
I think you have the stock suspension right?
You want either 245/45/18 (this is the same exact out diameter as 225/55/17) or 235/45/18 (slightly smaller). At stock height you can fit 245/45/18 easily.
You won't have the same tire thickness (but you will have the same overall diameter) switching to a larger wheel. You basically lose an inch of tire sidewall. This means that it should fit exactly the same as your current tires.
Where it get's tricky, is wheel offset is what mostly determines fit. Too low (et 0-30) and you could be running into the outer fender (doubtful on an Outback with OEM suspension). Too high, and you could be running into the inner fender (et 60+). All the previous depends on wheel width which I am assuming stays at 8 inches.
This is a really long way of saying you are fine with those wheels and tires as long as you get the diameter that matches what you currently have (245/45/18 or 235/45/18).
Tarmac 3s come in two offsets 18x8 et 35 or 48, plug that into will they fit and determine what you want. The 35s will be more HellaFlush (but, because of our flares will probably look normal as long as you have some camber), the et48s will be positioned basically where the OEM wheels are (still slightly more flush because of the 8 inch width and wider tire though)
On the OEM strut you should not have any issues with the tire or wheel running into it.
Look at this thread.
The first post by nightmare shows basically what the 8 inch width and et35 looks. The et48s would be sunken in more towards the car.