I haven't tried any 80 mph MPG testing,,, As I drive a LOT for my work, any more 'autograph sessions with the LEO's' would be 'very detrimental' to my career!
As aerodynamic drag inreases eponentially as speed increases, (not linear) I think, at least for the guys with better results at higher speeds, perhaps they have the 3.21 or 3.55 gears and may be just getting the Hemi up into it's most efficient rpm range, or, at a RPM range that doesn't 'lug' the motor.
The Gen 4 Hemi has a rather 'steep' powercurve, Kind of 'peaky' vs flat,,
I found it 'odd' that the 4.7 actually has more torque than the Hemi all the way up to 2600 rpm, and only slightly less HP at the 2,000 RPM mark:
My truck (reg cab) is also lighter than most everyone elses here, and the aerodynamics are probably a tad different than, say a Crew Cab,,,
With the 3.92's, my Hemi turns about (don't quote this as absolute fact!) about ~1900~ (?) rpms,,, With this set up, the truck is not 'lacking' for power at that speed,,,
The 'cyclone effect' is interesting,,, but if it means I'll have to drive 80 mph,,, eh, well, 'no thanks'.
Now, as far as the Tonno helping or hurting, I can't absolutely confirm either, but,,, Beaumont trip 1 was without tonno, and all the rest were 'with Tonno'
(Also, please keep in mind the truck had >1,000 miles on it during 'Beaumont 1")