In
this other thread I gave some results from fitting up 265/35R18 on our 18x9.5" +42 Wicked Awesome wheels all around. (Cliff notes: Front too big.) We do also have our 17x9" Awesome wheel, so as a follow up we fit a set of the 17x9" front wheels with 255/40R17, and the same 18x9.5" with 265/35R18 in back.
Careful observers will note that the 17x9" Awesome is a ten-hole wheel -- it was designed to fit any Subaru car, including ones with 5x114 bolt patterns. The 18x9.5 is drilled with only one bolt pattern, although it does have little holes between the bolt holes, so it's not as different-looking as it might be.
Both wheels are 56.1mm hub bore, so they didn't need hubcentric rings.
Here's how the car looks...

Blurry. Sorry.
I like the look. Makes me think of serious performance cars that have (or had) staggered wheels. NSX, C6 Corvette, Mercedes SL63 AMG...The smaller front wheel diameter helps to accentuate the car's trim front, while the bigger wheel in the back fits with the larger flank of the car.
Obviously a square setup is a lot more practical in terms of being able to rotate tires for maximum life, and handling balance may be better with a square setup, too. But especially for cars that are going to have much more horsepower than stock, a big rear tire is going to be helpful for keeping the back end attached to the road.
Anyway, I hope this is thought provoking, or at least adds to the general knowledge about wheel/tire fitment on these cars.
--Dan
Mach V