Seriously, I love the Epsilon chassis GM's (Malibu, G6 & Aura). We drive our G6 the 5-6 hours home to Cleveland several times a year, no issues. Ours has the 4 cyl. Ecotec and the 6 speed automatic. It will fly down the interstates with no issues and get 30+ MPG doing it.
The only problem I could see is if you got a SV model G6, apparently they don't have cruise control. But I don't think the rental agencies buy those. And if you have a co-pilot, that shouldn't be too big an issue.
I spent 6-1/2 hours, including a 1-1/2 hour nap in the driver's seat in my G6 on the way home in heavy traffic from Las Vegas to the greater Los Angeles area. No problems at all. I also drove the Hana Highway in Maui at a well-relaxed pace, mind you with lots of in and out of the car for photos, and it was comfortable for that as well. I also spent 2-1/2 hours last Graduation Weekend Friday on a normal one-hour drive. No problems due to discomfort in the car.