When I saw this thread I wasn't sure if it was tongue and cheek or if it was serious, especially after I saw the standing passengers..
So you want to chase efficiency? Ok...
Standing passengers, no drinks, one stew, only water is offered.
Single engine, big Williams, run it out as far as trend monitoring will allow.
Engine operated on long range fuel economy, optimized for winds/alts.
Single pilot operation, UAV pilot on the ground monitoring flight.
Simple avionics. No EFIS or FMS, basic two ILS heads, GPS, steam guages.
Reduce all weight, use of composites, fast wing, no TRs, no extras