My '97 OBS costs about $3000 a year to insure with no discount... That's with $2,000,000 liability, collision, and comprehensive. I think I have a $300 deductible, but I can't recall exactly, plus some other things like roadside assistance and whatnot. But, that's an older car, so a new STi will likely cost more to insure. I think they charge extra for the AWD bit (not sure why), plus they look at cost of vehicle, where you live, how often you drive it, etc. I know, for example, that it's about $15 a month cheaper to insure a car in East Van than in South Surrey... It may be more or less, depending on where you live.
In any event, ask the insurance place to price it out for you with just the bare minimum of insurance, then find out exactly how much collision and comprehensive cost (with whatever deductible you want), and how much the extra liability coverage costs, then talk to BCAA or CDI or whomever and see how much they want. It might be cheaper, it might be more expensive... It really depends on what you want in terms of insurance and who you go with.