Thanks to all for your feedback!
My current combination of Yoko Advan S4s in P235/50-17 on 17 X 7.5 et48 are at the limit of rear fender clearance. They did contact the rear fenders once when I had passengers in the rear and hit a sharp dip in the road at about 50 MPH. This is on Swift springs and - 1.5 degrees camber.
Tire selection plays a big role. The Advan S4s run really wide in the tread, with the side wall being almost straight up to the edge of the tread (as opposed to a more curved side wall on tires with equal cross section but with a narrower tread). The contact point on my current set up is about one inch down from the tread on the sidewall. With more offset (et44 vs et48) and a taller tire (26.4 vs. 26.7) on a 18 X 8.5 with P245/45-18, I am sure I would have major problems with a wide tread tire like the Advan S4 because (1) the tire is taller and (2) the contact point would be higher up - perhaps on the shoulder or tread itself.
If I were to do this, tire selection will be critical...