My situation? I am sticking with the stock 18x8.5" +55 wheels (cause they're free and nobody would give me decent money for em thanks to their mediocre stock tires) and I bought a set of 07 STi wheels (17x8" +53 clear brembos 5x114) to use in the winter with 245/45R17 Pirelli Winter Sottozero 3. That's not exactly the question you're asking though.
What would I buy for the track? The 08-14 STi could handle 265 width at near stock offset without fender rolling or hitting the strut. You can't find that width for a 17" wheel so if I was going to get new tires/wheels I'd be looking at an 18x9" or 18x9.5" wheel that cleared brembos and had basically as much offset as I could get. If you could find say 18x9" +48, I think you could run 265/35R18 and clear the fenders. The 2015 is new but the fender clearance appears to be somewhat similar. I think on the 2015 you could do the same.
The negative there is the tire height. The stock height is 25.7" and the STi's gearing is overly tight. 265/35R18 is just 25.3". I'm actually considering 255/40R18 on the stock wheels though strut clearance at that height (26.0") at +55 offset worries me. I'd actually prefer 255/45R17 if I could find and afford 17x8.5" +48 5x114 wheels and they made a decent tire in that size (which they don't so it's moot). 275/35R18 seems to fix everything but it's awful wide and requires an awfully wide wheel which I can't imagine having an offset that wouldn't make me puke. Also not sure how the fenders would handle it. 18x9.5" decent offset wheels and 275/35R18 tires is not a cheap option either. Or a nimble one for agility on public roads dodging potholes.
So all that said 245/40R18 on 18x8.5" +55 isn't all bad even if stock doesn't sound very ideal to most people.
The WRX, in all it's years, is a different balance of tire width, wheel offset, fender clearance, gearing, and brake clearance so keep that in mind if your car isn't exactly mine. For example, the 02-05 WRX came with 16x6.5" +53 wheels and 205/55R16 all season 25" tall tires on overly tall gearing. The difference going to a performance summer tire that was 235 wide and shorter (235/45R17) was biblical. The fenders were the same as the JDM STi to boot, which was 235 wide stock. 17x8" +48 to +55 offset 5x100 wheels are cheap and easy to find as well so there was no reason NOT to do it. As I said, my 2015 STi isn't going to easily slap on 30mm more width from a far superior tire for little money and no clearance issues like an 02-05 WRX could. It's pushed already.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD