They're still on the car, trying to pretend summer hasn't ended for another week or two yet though the tires are getting pretty close to the 3/32" 'no longer safe in the rain' wear bars and they're so old (I drive like 5k miles a year) that there's some dry rot setting in. I figure I can drive on the stockers/winters starting soon going into 2015 for a bit if I have to, I want to see what engine will be in the 2016 before I buy. My shit aside...
I usually tell 02-07 WRX sedan guys to get 235/40R17 on 17x8" +48. It's an easy to find choice and the tires are nice and short which really helps with the length of the 5-speed. It's also about as much tire width as you can push (and you need +48 for even that much) worry free, even on some of the crappier springs some guys run. It's affordable, easy to put together, won't rub, and drives great. Biggest negative is you will sometimes look at your tires and wonder if you could have squeezed more width. Those front fenders sure have some more room...
If you want to push things further (can't blame you I wanted to too), there are a lot of things you need to worry about. First, offset starts to really matter to the millimeter as you are rapidly running out of clearance from the strut and fender. 245/40R17 I'd say you don't want to go above +55 offset anymore for fear of hitting the strut, not that you'd find a wheel up there to begin with. Also 245 is really pushing it on 8.0" of wheel width. Tire manufacturers will often spec it but you're right at the 20% limit for 40 sidewall tires: 245/(25.4*8.0)-1 = 20.6%. Ideally for that you'd want 45 sidewall minimum but 245/45R17 is just too tall for the GD and would further exasperate your tall-ass 5-speed and good luck finding >8.0" of width without cutting too much offset. Speaking of offset, another complication with 245 is you are running out of fender clearance too. Lower than about +48 you'll start hitting the rear fender. This is reasonable to fix with a roll and nothing too crazy (guys do that all the time for 17x8" +45 RPF1's with 245/40R17) but it's a pain when the rear is weighted down (passengers, trunk). Furthermore, 245/40's are taller than 235/40's and adding more tire width starts to really affect the turning radius. The friction to the road starts to make the steering stiffer as well.
You can always go further than you should. 255 will not clear the rear of the car with stock struts and fenders at any offset... barely. The LEAST fender modification for this size is +48 IMHO. You're EXTREMELY close to the strut (but tires I've checked do clear) so you can't raise it more and you'll still hit the rear fender lip when there's a big bump. The rear fender needs to be completely rolled, though the fronts are fine. 255 is stupid wide for 8.0", really need 8.5" minimum. There are very few wheels to choose from 8.5" or 9.0" wide especially considering the 02-07 WRX is 5x100. The most common choice are the Rotas which are 17x9.0" but a scant +42 offset. That's 6 more mm of rear fender that needs to be removed from the +48 and the fronts probably need a roll at that point too. If you're crazy like me, you can track down a few wheels that were made 5x100 17x8.5" +48 specifically for maximizing tire width on the GD chassis. They are very difficult to find and you still have to deal with the rear fenders. Furthermore, 255 is quite wide for a WRX and the height advantage is gone (now slightly taller than stock WRX size). Your turning radius will increase by about a car lane from stock (not exaggerating this trust me) and will resemble that of a standard pickup truck. Steering is stiffened to the point of requiring 2 hands to turn, especially if you're using a high friction tire (read: grippy).
Further on for 265/275, 8.5" of wheel width will NOT going to cut it. You'll probably have to switch to 18x9.0" so you can run 30-sidewall (potholes ahead!) and stay <25" total tire diameter. Offset has to go out from +48 half of the added tire width from 255 minimum (example, +38 max for 275) and your stock rear fenders must be removed and replaced. I doubt the front will be much better off. 285/295 can be done too but you're not going to be able to cut sidewall any longer so you also have to deal with added tire height. I'd recommend a full widebody kit, a custom set of coilovers built for such a radical change in center line, and a transmission that's not so damn tall. Sanity was not mentioned in this paragraph. Street cars have been left at the apex a good while back.
I went with the 255 craziness. The big driver for me was wanting to avoid the shorter tires since my 04 STi's 6-speed was so damn short, opposite of the WRX. 245/40R17 seemed too short (not too narrow mind you) and 245/45R17 was not going to clear anything. Note the 08-14 STi had much the same transmission but 245/45R17 was the new STOCK tire height (though it had 18's to be cool yo), worked much better. Pushing things to 255 gave me more height, precisely something GD WRX's SHOULDN'T want. After driving it around for a few years, I have to say it's overkill and my rear fenders have never been happy with me. My tire replacement plan was to find some 235/45R17's (Stock JDM GD STi size) that ran W*I*D*E like the Federal 595 RS-R and go from there. I'd gain some height, probably not loose too much width, and hopefully keep my fenders happy.
From a WRX perspective, you have incentive the other way for shorter (not narrower) tires to make your 5-speed happy. I'd recommend even if you want to push things sticking with 245/40R17 as a maximum. It may be the absolute widest tire you can find that's under 25" total diameter outside of some custom jobs best not to think about. 255/35R17 or 255/30R18? Don't exist. Height aside, wide tires are great on a racetrack but make the front wheels more of a pain for driving around town. I'd still note the value of an extra 0.5" of wheel width if you can track it down (the mythical 5x100 17x8.5" +48) to help the 245's ride with less sidewall flex. The 17x9.0" +42 is a good match for the 245/40R17 size except that your rear fenders are in serious trouble. Also, +42's a LOOONG way from the stock offset which will compromise the suspension's performance quite a bit.
Not trying to sell you mine but figured I'd offer. I can flip them easy on nasioc so don't worry about hurting my feelings if you don't want em. I'll try to get the car washed and take some pictures of em but it might be a week or two.