I'll reiterate this (and probably cut and paste it as a disclaimer). Consider it my wheel and tire primer if you're going to ask my opinion; I want to help people, but I really, really, really don't want people to be disappointed with new wheel setups when they don't fit without a bit of work. I've run a ton of different wheel and tire setups, and a ton of suspension setups. However, without exception, every set has been on aggressive suspension, and I rolled and pulled my rear fenders from the very start, because when I was running my Ground Control coil setup, I RUBBED WITH 215's IN THE REAR.
That's this setup:
17x7.5 Gram lights, ET48, 215/45/17 S03's. Ground Control coilover sleeves with 250lb progressive rear springs and 315 fronts. Those rates are stiffer than most lowering springs, and you can see the car isn't slammed. I've since moved to Tein Flex's, and soon to Cusco Zero2R's. I run much bigger setups:
....but they're not easy to fit on the car. That's an 18x8.5 ET48 with a very narrow 235/40/18 (more like a 225).
I've never run lowering springs, so I have no idea how much I would have rubbed on them. I never ran wide tires without coilovers so I have no idea how much I would have rubbed on them.
That said. If you want to run a beefy wheel and tire setup, run coilovers. End of discussion. You can adjust ride height, you can raise the back if it's rubbing over bumps, you can change spring preload, you have more space on the inside, etc. Firmer rates will let you keep your wheels away from your fenders, but keep in mind that you're, well, keeping them from moving. Subarus handle rough terrain and inclement weather because they have lots of wheel travel, in addition to great balance and AWD. It's a tradeoff; my cars don't handle demolished roads well. They don't like bumpy gravel. Suspension is meant to keep your wheels on the ground at all times....and stiff suspension can't do that on rough terrain.
So, that said, I can run a pretty mean setup on my car with the suspension and fender mods I've done. I don't rub anywhere, over any bump, in any corner, with a 235/40 on a 17x8 ET48. With a 17x8 and a 245/40/17, I buzzed the front fender liners just a touch under hard cornering. That is on, as most people will agree, a pretty heavily modified car.
If you are not willing to roll your fenders, don't run 18's.
If you are not willing to roll your fenders, don't run 17x8's or wider.
If you are not willing to roll your fenders, don't run over a 225.
If you are not willing to roll your fenders, don't slam your car with anything other than stockers. Something else might work, but I'm not going to be the person responsible for someone spending hard-earned money and being unhappy.
My wheel fixation is not natural. Normal people don't spend 25% of the book value of their car on a set of wheels, let alone have a garage full of them. Find someone like me in your local area once you get your suspension. Be nice to them. Ask if you can test fit a wheel in their driveway so you can see if it has any chance of fitting. Understand when they say no and thank them for their time. Be patient. Don't rush car stuff - a better deal will always come along.
Other facts of the tire world that are very, very important.
Tire width is not absolute. This is the most important point I can make. A 225 from one manufacturer is not the same size as a 225 from another manufacturer, or even another tire from the same manufacturer. STi OEM tires, an RE070, that's labeled a 225/45/17, are as wide as most 235's, if not more so. The "235" Nexen's I had on the Slipstreams were wider, on a 17x8 wheel, than a 245/40/17 Dunlop UHP tire on 17x8.5's on my STi. Their 225's are the same way. A Falken all-season 225 is narrow, a Falken RT615 is wide......I'm talking at least 1/2" difference. Measured section width is the only thing that matters when ordering tires; you can find it on tirerack, vulcan, edge, etc. Ask, and look around at meets, too; you soon see what tires run narrow or run wide. The most important thing is, run what fits your car. There's no reason to cram a 235 on an 8" rim on your DD with no suspension; you'll get less steering feel, more body roll with the increased grip, rubbing, etc. Run a 225 on a 7.5 instead, and save yourself from heartache.
I always stick to this idea; you pick the component most important to you, then you make the necessary other mods to keep the car functional/safe along with it.
This means compromise. Set A as a requirement, then figure out what it requires in systems x,y, and z. Do them along with A if not prior to it, or you're setting yourself up for failure, damage, unsafe vehicle, etc. Do not halfass any parts that affect the safety or control of the 3000lb+ weapon you're trusting your life to.
You want to run wide wheels and tires, or an aggressive setup, etc - better plan on fender mods and stiff coilovers so you can keep them off the fenders, and you need suspension first.
You want to run the car low, you run a narrower wheel or tire setup that will tuck in easier.
You want a cushy ride, you set up your suspension first and then figure out what wheels fit under it.
"What fits" is a relative question for each car and its intended use - there isn't an easy answer, which is why we ask so many questions in response. I hate to use a cliche, but when dealing with aggressive wheel fitment especially, if you don't know, you shouldn't be trying it, especially on a small-market car that doesn't have a huge tuner base. Only one way to learn, to be sure - but you need to understand the fundamentals of offset, wheel width, suspension clearance, camber curves, scrub radius, etc. It all matters. I can look at the specs of a specific wheel and tell you exactly where it's going to sit in the wheelwell. I can't tell you if it's going to touch your fender under your conditions. You need to understand what the measurements mean to you and how they affect your car.
Put into perspective, I know exactly what fits on my car with my specific mods, spring rates, camber, ride height, sways, driving style, vehicle loadout, etc. How do I know? I've had 37 different sets of wheels and tires on the car, from 205/45/16's all the way up to 2435/35/18's. Still doesn't allow me to say "this will fit on your car", because I don't own your car, and in many cases have never run a setup like yours. I've never run lowering springs. I went to RS wheels on wide tires years before I lowered my car on a custom setup - custom as in the shop I was working for was owned by the guy that did kit design and fitment for Ground Control back in the early days. I've *never* had an off-the-shelf component (besides OEM) bolt onto the car and work without mods with the exception of a Legacy-specific front sway bar and an ALK. I want to help, but my experience is limited with stock-type suspensions. The last 5 Subarus I've owned have either been on Tein Flex's or a custom Ground Control setup, with no exceptions, and put on that suspension prior to playing with wheel fitments.
There's my rant for now, I'll add more later. Sorry if my fat fingers made some typos