235/40-17 will be fine on that rim. It will be a bit shorter though than the stock size. The speedo on the STi's are already too fast with the stock tires, that size will make it a bit slower. The 235 tire might be a pound lighter in some cases, but it is a pound heavier in others. Not enough to make a difference. There will be no difference in performance in the rain as well for that little a difference. Never known of anyone having insurance problem with larger tires either.