Actually the C6 is pretty comfy inside. The wife and I take long week end trips in mine and go for hours in comfort. Could the interior be better? Of course, but its really not that bad.
Mileage? I cant think of another engine made by anyone who makes that kind of hp/tq #'s a LS7 gets w/o getting a gas guzzler tax stamp. The fuel efficiency of the LS series engines is phenominally good compared to others.