225/40/18 is the closest match to 225/45/17 in terms of matching the rolling diameter. So that should not be a big problem, but ya i think ET32 is definately pushing it in terms of space available under the rear arch, that's sticking 3mm further out than even the GTA rim. I am using the 17" Selespeed rims, which are ET40.5, and even then the clearance with the outer liner at the rear is not huge.
Lowering the car might make your situation worse, but there again if the problem is happening over bumps etc stiffer springs might not be any worse than what you have now.
I'm sure there are people out there on 18"s that can help more.