If you're going to do a lot of highway driving I'd really look into an ltz or other turbo m6 car. The rs will do it fine, but 70 mpg is about 3000 rpm, I don't know off hand what other models are but I'm sure it's significantly less.
On the upside, you rarely downshift. My car is tuned but it will pull most hills in 4th over 20 mph, and around 30-35 you can use 6th pretty well