He said he actually has better fuel economy with this engine than with the original, because he can quickly climb where air is less dense and cruise at that height at low throttle.

It probably helps that the original Wilga is very fuel-hungry, it's basically a flying tractor :)

It's really not that unusual these days...

Consider the Lancair Evolution https://www.evolutionaircraft.com/

Turbines can actually acheive better specific fuel consumption (lbs fuel/hp/hr) than piston engines when operating at high power... it's actually at _low_ power settings that they really suck fuel wise.

If you watch the videos he adds some fuel capacity. The particular engine he's using isn't horrible on fuel consumption, particularly at altitude, though definitely worse than the original IO-540