I believe this is covered in the Johan Andersson Battlefield 3 Presentation tech talk at the NVIDIA GeForce Lan 6. He indicated that no settings are ever so low that they confer an advantage (e.g. disabling all trees, etc.)
It's in part 5 at about 1:15 seconds in.
In a multiplayer game you can't just disable everything. Because that increases the visibility too much. If you disable all our particles and all our shadows it becomes like Quake 3 arena. And that's not really fair.
The other parts: one, two, three, four.
I would shoot for mostly High settings, as that is what the designers intended. (I do recommend turning shadows to medium and avoiding MSAA as the perf penalties are far too high for very little visual improvement.)