I have the same problem. I have 17x7" rims with +40 offset and 205/45/17 tires. After rolling the rear guards, there is still some slight rubbing when there are 2 or 3 people in the back seat and when I drive over a big bump or pot hole. Looking more closely, the rear tires are actually rubbing against the corner of the rear bumper, where the rear bumper meets the rear panel.
I'm planning on lowering the car soon, so to fix the rubbing problems, I'm planning on modifying the rear bumper slightly by either:
1. grinding/sanding down the inside of the bumper where its rubbing, kinda like rolling the bumper part of wheel arch, but actually cutting the plastic instead of bending it. Maybe i can actually bend it if i get it hot enough?
2. pulling the bumper out sideways so that it flares outwards more, and won't get in the way of the wheel. The bumper is quite flexible, and there is a bolt near the corner of the bumper that mounts the bumper to the chasis, so I might be able to drill out the mounting tab for that bolt and put in a spacer to push the bumper out sideways. kinda hard to explain without pics...
As a last resort the tyre shop say they can machine 2-3mm off the mounting surface of the wheels which would change the offset to +42. I wanted this done straight away, but they recommended against it since it voids the wheel's structural warranty. But they said its fairly safe to do the machining. Another tyre shop also said its pretty safe to machine 2-3mm.
Hopefully this will fix my rubbing problems, but my wheels are only 7" wide. If yours are 7.5" this might not be enough. Will let you know how I go.