One thing that struck me when trying out X-Plane 10 was the varying sound the engine makes when you change the position of the collective.
Really, really immersive and very different from the constant/steady engine sound we get in ToH!!
Looking at various youtube movies, I think there is indeed room for improvement with the engine behavior and related engine sound of the ToH choppers.
Have a look at this movie clip.
Hopefully RLIB4 will make it possible to implement these improvements.