That setup works fine, ran it on my 04 WRX. You can run 15" wheels, you can run 20" wheels. To the suspension, they're pretty much the same. You just have to buy tires that are tall or really short accordingly to keep your stock dimensions in terms of height. WRX guys should generally look at tire heights between 24.5" and 25.0"
In terms of width, each car is slightly different but all can fit wider than stock tires. You're not adding any tire width, just shifting 7mm (the stock offset is 55 and those wheels are 48) so you'll have 7mm more clearance from your suspension and be 7mm closer to your fender well. This isn't ideal, but works fine. +48 is a nice offset for any Subaru.
Switching from a 7" width wheel to an 8" width wheel but keeping the same tire height will tighten up the sidewall some but will hardly change the overall dimensions of your tire. It might bulge a mm or two further out on the sides depending on the two tires you're comparing but it's not a drastic difference.
I'll also give you my personal opinion that 18" wheels don't really make much sense for a car with 25" tires, especially one like the WRX which really benefits from a shorter tire (shortens the gearing). I always try to get every WRX owner to try 235/40R17 on a 17x8" +48 wheel to start. Compared to 225/40R18, it's got more sidewall (no stock Subaru was sold with a 3.5" or less sidewall), more width, and better gearing. They're 24.4" tall which is pretty much perfect for a WRX IMHO. They do make those OZ's in 17x8" +48 too fyi
