In the AI mind...
It does impact a lot, yes. GPU is not that much affected by it in fact, it's a CPU thing more than anything. The less objects/triangles, the less information to carry, the better. Simple as that (or close to). Especially if the track is already pretty demanding
The Kunos mod guideline is actually quite good to help for optimization (dev folder, like the kseditor). It's not a formal rule, but it helps. LOD C should be minimalistic as much as possible, LOD D is close to a placeholder (for some cars, 1 or 2 object/material is enough), as it is not meant to be seen "clearly". LOD B is the tricky one, the transition can be hard to get right. But the general rule is : if at a driving/replay point of view it is not visible, it shouldn't be there at all.
Edit : as a reference, what I usually do for the meshes :
- If the object is not visible/useful anymore, too small or hidden in sight : get rid of it
- If the object is part of a general thing/do not attract sight : decimate from 0.2 to 0.1 per LOD, join per material as much as possible
- for main objects : clean-up manually and/or decimate from 0.5 to 0.2 depending on the result, each one is different...