Moving back to the topic at hand, most people want distance nowadays, a less effecient engine causing the plane to go 633nm may not be what most people want, with 100LL getting more espensive, people want cheap fuel and they want to burn less of it. Put a more effecient engine in that plane giving it more range (1k is nice) and potential sales will no doubt go up. overhaul costs may be higher in a piston engine, but when you burn less fuel, it may be worth it over 2k hours as opposed to 3k using more fuel.
just a newbie diesel lover's perspective.
(distance AND speed)
Just thought i would add something about mission weight. An engine that burns more fuel will need more of said fuel for the same trip, possibly offsetting any benefit of the lighter engine. So it is quite possible that a piston engine producing the same hp as a lighter rotary will have a lighter mission weight (say to travel 800nm). I'm not completely discounting rotary engines, heck they seem like a really neat engine that would be amazing for twin tractor operations (small cowling) but i like to go far.
David