UrS4

There is no reason why you can't fit a 285/40/18 tire on a 18x10 wheel with a 40-45mm offset which is what most of the offset's are for the S197. Just measure the Roush wheel, you can figure out where to measure from with any of the wheel calculator web sites.

You can fit a 305 tire in the front, Griggs does it but they switch out the suspension goodies.

I'm a little perplexed by your question, why would you go narrower when you have a 275 tire on a 10 wide rim with no rubbing. If the Roush can do it on there Stage 3 and track pacs and they are lowered, why can't you?

Texotic

Griggs switches the front suspension to an SLA setup, which allows them to fit the 305 up front. You can fit a 275/40/18 on a 18x10 on the front with no issues, even when lowered.

Hootna

If it was me I would run 18x9.5s all the way around. I wouldnt expect Roush to know crap about their wheels. Am sure they made sure they will fit and have put them on the track for testing but after that just care about pumping them out the door.