I think there is no foundation for your assumption that someone driving the speed limit will necessarily be paying more attention than someone exceeding it. Someone who is truly a "jackass" may indeed not be paying enough attention, but he's as likely to be a jackass because he's dawdling along 15 mph slower than the flow of traffic, or talking on his cell phone, or a combination of factors.

It's safer to move close to the speed of the flow of traffic.