You don't need 18" wheels to clear the STi's brakes. There are plenty of good 17" wheels to choose from for the winter. One excellent choice I always recommend is used 05-07 STi wheels. They're 17x8" +53 and work very well with 245/45R17 winter tires for the 08+ STi. Subaru friendly offset, BBS looks, lightweight, and can generally be had for <$600 a set.
Note, your stock wheels are 18x8.5" +55 with 245/40R18 SUMMER ONLY tires.
Snow tires, also known as "winter tires" and "ice tires," are tires that have been designed and manufactured specifically to perform well under winter conditions. Snow tires are different from other tires in both their tread and the kind of rubber that they are made of. The rubber that is used to create snow tires is softer than tires that are used in summer conditions. The softer rubber grips ice and snow better than other tires. The softness of these tires is not ideal for summer months, however, and in moderate to high temperatures, snow tires will quickly wear out.
The tread on snow tires is also much different than the tread on other types of tires. First of all, snow tires have more grooves than most others. Like the soft rubber, these grooves also help the tires to grip snowy and icy roads. ,..copied Real Estate Marketing