Not that this will plush a Mustang out to the level of the luxury sedans you'd been looking at, but if you end up deciding on a '14, one of the extras that came with mine was the 401A interior upgrade package, which I have found I really like.
Over and above the Premium trim, it adds different leather with contrasting racing stripe on the seats, door panel inserts similar to the GT/CS, darker metal/machined pattern dash paneling, and an aluminum shifter ball. It's no Bentley but the subtle extra touches definitely add something to the overall experience.
Black 2013 Mustang 5.0 MT82 w/401A package and 3.31s
Black 2000 Mustang GT (owned 2001-2013, sold at 177,000 miles)
Brown 1971 Triumph TR6