I can't see it being related to a straight through exhaust. If your exhaust is not leaking this would be a general issue with all sapphs, which I'm sure it isn't. Over the years I've done about 230.000 km in sapphs and I've never noticed anything like it. The only time I really smelled exhaust gases in the car was when the exhaust was broken in half right underneath the diff. At least that proves that if it's leaking at the rear silincer joint you can still get fumes into the car.