I had 245/45-17s on stock 05 sti wheels and swift springs. I rubbed the left front every day when I made a hard right at the bottom of a certain hill. That was the only situation when I noticed it rubbing. I did trim the bumpstops, that may have been a factor. The tires looked noticeably bigger. I don't think 245/40-17s would look noticeably smaller.
Edit: That tire wore out way too fast. I would never buy that size again.
Edit again: I also have a GT Spec ALK. That could also be a factor I hadn't considered.