Is your car a sedan or wagon? The wagon doesn't have much fender clearance. 17x8" +48 would not preset rubbing issues for either car, with reasonable tires. Doesn't rub inside or out, not even close. If you have a sedan, I'd recommend 235/40R17. They're a little shorter which is good and also a nice increase in width. They're also pretty square with the 8" width. Unfortunately, they're just too wide for the rear fenders on the wagon. If you have a wagon, I'd go with 215/45R17. 225/45R17 would work on either, I just think the WRX is better off with a shorter tire. I'd actually recommend 225/40R17 over 225/45R17 for the WRX though cost may rule that choice out.
If you want a lot of tire width, offset is key. There's a lot of clearance between your stock wheels/tires and the struts. Fitting the maximum width tire will require a high offset wheel.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD