5 hours? Is that all? ;^)
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.