The only times I can think of needing to downshift at 70+ mph is 1) Emergency 2)Track racing and 3) driving like an asshat. In the time it takes me to rev-match, downshift and floor it, gaining the necessary speed to pass, I can just put the pedal down in 6th and be going as fast as I need to, frankly. Might as well save your clutch if you are just on the highway.
p.s.--the fuel does not come in contact with the transmission, which is what I think @
Tcoat is trying to get at. The assertion that E85 somehow makes your shifting smoother is pure butt dyno.
EDIT: This is with my stock FR-S. Using e-85 or FI, even a tune might change the most effective parts of the powerband. That said, still seems unnecessary to me.