At the moment, fuel efficiency vs speed is part of engine design. It’s a strategic decision and you need to create a variety of different engine designs for different roles and situation.
Allowing a range of fuel efficiency options within a single design removes that strategic consideration element, so I won’t go down that route.
The option I am considering is an engine modification that has extra cost/weight and provides a limited ‘tactical’ benefit in specific situations, with the added possibility of a negative tactical impact. That would still allow a strategic decision as part of engine design and add some tactical decisions under certain circumstances.