FYI, I am running 18x8 in standard Rota offset {48} I think, with 235/40/18 and it clears fine. I think 245 will clear without rolling as well. I will find out once these tires are done if I still have the car.
FYI, 3gwrx.com is a useful site for 08+ imprezas. Not as many dick heads there as nasioc too.
Our imprezas has a bit more room for tires than the 02-07 sedans did. Not a lot, but a slight bit.... Definitely wider than the 02-07 wagons.