It is hard to say.
TS Performance advertises 20% fuel savings.
The guys I sold the units to for the six row pickers said there chips were paid for in 5 tanks of fuel.
The pickers are speed limited by the hydro's and sync with the drums turning in the heads.So they can't really push the speed past a certain point.With a combine you can push it faster so you could possible use the fuel savings by overloading the machine past capacity.
So if you used a little common sense and didn't overload the machine past it's capacity you could gain some useful speed and also save on fuel.
Just my two cents.
Maxie