I just want to run them because they are cheaper (when take-off are available), and I already have a set

I'm lobying to use the ST rules for our T* modifier, mainly because I already bought tires and am lazy.
In reality, though, I could always pick up a new set of tires to run on, but I don't exactly want to. Or, I could always just run in SM with the rest of ya'll.
But here is a question. The Legacy Spec B comes with RE050's in 225/45/18. Iff I could find a set in 225/45/17, would those be legal to run, since the tires comes as OE on a version of my car, but in a different size?