Great response, thanks, caused a number of pennies to drop.
To elaborate some, I'm starting to make some longer trips (rather than the $100 hamburger runs), and aiming to plan better alts. Now, I'm paying for the plane by the hour, wet.. so my main aim is to fly faster - but on the other hand, I'm also a bit load limited, so burning a bit more efficiently helps - both to have more margin from what I'm carrying, and to potentially carry a bit less.
Would I be right in thinking that the drag side relates to IAS, rather than TAS - so hypothetically flying high where the IAS is significantly lower for a given TAS moves us into the more advantageous part of the drag curve - at the tradeoff that the engine/prop efficiency reduces?
Cheers,
Mark.