Along the back stright with all the open area there are actually alot of individual trees, as well as a lot of tree wall. The pines before the viaduct and the willows after the viaduct are all individual trees for a certain distance, but by the time they are 3-5 trees back from the road i have some tree wall. The shadows i've mostly set to medium or low in those areas. I had tried with more or less tree walls, and infact in the willow areas i did initially have some tree walls, before i knew that they weren't as dense as i had them, but the walls themselves had lots of zig-zag in them to make them look a little more alive. I found that beyond some point they slowed the framerate considerably. as soon as i removed them the fps went up a noticable amount. they did have reasonable shadow settings too though i think.
I think the way i tried to judge it was to have the tree walls only close enough to the road so that i don't notice them as walls. It also depends on how many polys are already in that part of the circuit.
In short, the way i've done it is with areas that have lots of dense trees i have 1,2,3 rows of trees then a wall. Then i try to keep the clip distances as low as possible without pop-ups. And if it's still no good i lower the shadow levels, shadows fall on cars and recive light setting.