You seem to infer the Lycomings and Continentals are inefficient. Do you have facts to back that up?
No I don't, because there are no other engines out there to be compared to.
You say that there are no alternatives because they are the most efficient. Maybe so, but at least allow me to be a bit skeptical when there are no others.
I say there are no alternatives because of too much money involved with too little return and too much liability.
I know you can only extract so much energy from a liter of fuel, but I'd like to see others trying and maybe failing on technical issues, instead of paperwork and liability barriers.
As I'm sure you know, Rotax had a V6 in the 200 hp class in the work, and decided to kill it for those reasons.
Was it less efficient than an O-360? I guess we'll never know.