Track looks fantastic.
I'm for billboard trees as well. Or as an alternative you could render the 3d trees with the normal maps and fiddle with textures so the billboards of them match the 3d trees. Then you can do more faraway trees billboard and leave trees close to track as 3d. And use the rendered billboard tree as a LOD for the 3d trees. This way the self shading wouldn't appear out of sudden and far away trees won't look flat.
Also shadows from them can be made of invisible shadow objects, simple X-shaped sprites, this way it would take down way less FPS.