The Piaggio II (https://www.aircraftcompare.com/aircraft/piaggio-p180-avanti...) is missing several modern aerodynamic and efficiency features like winglets and gets 15MPG and has a 1600 Mile range. Significantly beating it’s fuel efficiency at similar speeds seems reasonable just eyeballing the two designs. Getting the full 20MPG at 450MPH might be a stretch, but it’s not all that unreasonable.

Also, higher aerodynamic efficiency means lower HP requirements. Moving that weight savings to fuel can boost range even further.

PS: Winglets are actually really old technology. It’s making the trade offs worth it including significantly increased weight at the wing tip that’s new.