Surely it can't be any more expensive to insure an STI than a modified WRX running 280bhp?
And your budget may need to extend to a new engine: AFAIK the WRX unit is good for about 320bhp tops, then something important mechanically is likely to let go. If it's not the engine, it will be the gearbox. Go to 280, then start saving for the STI.
M