I don't think # of polygons is very important at the moment, relative to other things - eg. screen space shaders should run about as fast on any track. Using simple fragment shaders on the main visible materials might help.
Seems to be true, I'm running a new 320K scene, with rocks, my custom xtrees, etc...along with some good shaders, approx. 2400 distributed objects.
Also, I've been testing the road vertical random noise function thru BTB @ 0.05 first & later @ 0.01m height which represents too much rumbling imho. It's better to tweak it via special.ini instead something around 0.001-0.005m.