I can tell you that I have almost 25k miles on my truck and still only see 11.5 - 12 mpg. That's the same milage that I've gotten since day one when hauling empty and that with the cruise set at 65.
When I pull a 16' enclosed work trailer with minimal inside, I get about 10.5 @ 65 and if I go up to 70, I get about 8.
If I'm hauling my 38' enclosed work trailer with heavy load (trailer and load is about 25k+), I get about 8 @ 65 or 6 @ 70.
Bottom line is I don't see an improvement coming anytime soon unless it's through the purchase of an intake, tuner and cat dpf.
I drive almost 1k miles a week sometimes and I spend over 300 in fuel. I could run a Spartan 150 or 210 tune, get an extra 4-6 mpg out of the truck, that works out to 130-190 more miles out of a single tank, the savings I'll have can go into an interest earning saving account for when the truck does finally break down.
Bottom line is, if you are driving an F450 daily it's going to cost you. It's just a matter of how much now and how much later...