With the 2011 Outback like I have you could only get the CVT with the four cyl. We usually travel light
so gross weight
might be a factor. As far as the roof box goes I had a Jetta Diesel that got 49 mpg on the interstate. When Installed and loaded up (120 lb) my Thule long roof box I would get 52 mpg. with the Jetta. Like you said go figure.
BTW my Outback averages over 30 mpg but it's about 100 miles to the nearest large town, I do mostly 4 lane highway driving about 60 mph.