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The biggest limiting factor with GA aircraft is the old engines they use. The PA31 we use (Piper Navajo) has two LT540's. That's 540ci (8.8L!!!) turbo and they make 350hp. We're talking like worse than the average american V8 there! The turbos keep the power up as you climb.
Most piston AC engines were designed in the 1950's-1960's. They're simple air cooled 2v pushrod engines. Usually really low compression and commonly carburated because of the cost of certifying an EFI system.
The heads are about as good as you grandfather's 3.5hp briggs & straton lawn mower engine and they use huge quantities of fuel to keep the engine cool. The cylinder balance is horrible (they run per cylinder EGT) and they run 2 sparkplugs per cylinder.
They do not meet modern standards of efficiency or power output but nobody can afford to design,build and certify a modern replacement. I'm not sure about home-built or experimental, but the AC I'm familliar with have a TBO on the engines so every X hours you need to remove and rebuild them. Goodbye $75,000.
Since they have mixture adjustment and EGT/CHT probes there are a number of manufacturers who have approved an enleanment cycle during cruise to drop fuel consumption.
I think it would be nice if they could update the engines with a modern 4 valve OHC engine that is EFI and could run on non lead fuel. AFAIK you can't even buy unleaded at the pumps. 100LL is the best we can get.
Best thing I've seen so far is a turbo diesel conversion for some cessnas done down in texas I think. About $120k and it burns 1/3 less fuel and you fuel it on JET-A.
Sorry, but aircraft are not the pinnacle of efficiency.
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