Using a mciro gas turbine as a Turboshaft or turbo prop is again very very cool, but extremely inefficient. The power extraction turbine and gear box are reducing your system efficiency. This is balanced against the improved propulsive efficiency of the large diameter helicopter rotor, meaning you can now lift the engine, it's fuel and it's payload vertically. In a model helicopter, the gain in coolness offsets the terrible efficiency of having to land every three minutes to refuel. This is a niche market.

You really can't get around the fact that turbo machinery doesn't become competitive for flight until you get to continous power of about 400-600HP. By that power output, your recips are getting so heavy, that the mission weight of a the turbine plus fuel is less than the weight of the heavy but efficient recip plus fuel.

Reliablity, and cheaper, lower quality fuel are further reasons why gas turbines have come to dominate from about this power range and above.

Below there, you can not (with current materials and process technology) beat a intermittant combustion engine.

A gas turbine is a continuous combustion device, so as it gets smaller, the combustion chamber has to shrink. The CC volume reduces at the cube of linear dimensions, while the surface area is only reducing at the square. As you ratio of surface area to volume increases with reducing size, your heat losses through conduction increase (minus efficiency) and your skin friction losses increase (minus efficiency). In the turbine and comprossor, your tip clearances reach a practical minimum, but you are using small blades, to the relative tip losses increase as well.

An intermittant combustion engine is also affected by these effects, but to a lessor degree, as you can reduce the number of cylinders, and still keep the CC volume resonably high. The Cylinder is sealed, so your tip losses aren't an issue, meaning you can keep pressure ratios up even as size reduces. As efficiency remains high even down to very small engines, while the weight drops dramatically, your small recip plus fuel load, can be far lighter than your small gas turbine plus fuel load.

There was a NASA study in the late 70's which idendified the Wankel as the most promising technology for low power GA aicraft. It's combination of high power to mass, high power to frontal area, soft failure modes, and low vibration are all favourable for A/C. It is a shame that the bottom fell out for the GA market just as Curtiss Wright etc were prototyping new wankels.

The power level where gas turbines become the most attractive solution will creep down as breakthroughs occur in blade coatings and small turbomachinery design, and trickle down.

A real revolution can probably only occur if monolithic ceramic rotating assemblies become feasible. The hotter you can run your turbine inlet temps, the higher your efficiency. At present, once you get below the about 10000 HP, your turbine blades are too small to support internal blade cooling like the bigger engine have. Therefore your material becomes the limiting factor for efficiency (a big commercial engine can run turbine inlet temps above the melting point of the blade material, as the blade is internally cooled, shrouding in a cooling film of cooler air, and has a ceramic coating).

Metals are already pretty much at the physical limits today, to make a signicant jump in temp, ceramics are the holy grail (this has been know since the war - RR, GE and P&W have investing millions in trying to reach the goal of a ceramic which can survive the heat cycles of repeated starts without shattering as they cool).

The laws of physics shall be strictly inforced!