onetrack
Had you ever considered that the current crop of avgas burning aero engines may well have always been as efficient as you can make a a petrol engine. There were no appreciable gains to be made.
Have you every noticed how almost every automobile to aero conversion has been so totally unsuccessful?
Perhaps the current crop of aero engines isn't so bad after all.
I suspect much of the automobile gains have been in technology gains with respect to the whole motor vehicle, i.e.some engine improvements along with gearbox and drive train improvements, weight reductions, more aerodynamic shapes etc.
It's pretty hard to make significant gains with your average present day aircraft. Physics dictates the shape/size etc.