Hi Christian,
so you seem to be better equipped than I am! And probably more experienced as well… 
But I am glad if my rambling helps!
So do you have difficulties with the tarmac due to uniform color and that is why you need higher detail calculation?
Then you might try to follow the track at a lower altitude next time so you get distinguishable texture?
As I understand it, you are not primarily interested in the polygon or vertex model but the heightmap, which is nothing else than a raster image, right? So your problem is not the high polygon count per se but rather that it is one of the steps to achieving the heightmap?
So what you really need is software that can produce a heightmap from a large point cloud, right? I never used CloudCompare, but as I understand it, that is one of its features. Have you tried that yet? I looked real quick and it says it can do 2 billion. No idea about the performance though…
Good luck!
Götz