As light as it is, you'll take a hit to the MPG with the 4.0 and an FWC because of wind resistance and weight.
That's one of the main reasons I went with the FlipPac - weight = 290 lbs
.02
I do not have the numbers from FWC yet, but I am hoping the Finch shell would come in around 450 lbs. (The Finch weighs about 650, the Finch Classic weighs 550, so I am hoping the Finch shell is in the 450 range.)
I like the FlipPac, but I just can not get over the fact that it needs a separate rain fly.
As far as wind resistance goes, I have a good bit of it now. I am not sure how much the non-areodynamic square shape would decrease mpg from what I am getting now. A 1 mpg decrease would be okay, 2 or 3 would start to hurt. I would get better mpg during daily driving because I would have an empty truck bed and roof.