Here's the real math...
225/45/18 is 25.9" in diameter
245/40/19 is 26.9" in diameter
The difference is one inch. It also means you make your center of gravity taller by 0.5" and increase ground clearance by the same amount.
And, you have probably made your car slower overall...acceleration, braking, turning. Is this what you want to do?
I will say that there is not as much information out there for the Legacy as there is for other cars like the WRX or STI.
Good luck whatever you decide to do.