Yeah, I agree that LODs can make a lot of difference, I'm probably just over reacting.
Actually, I just decided to do some testing in pCARS, played with 64 cars in LOW settings and on ULTRA settings, and in the end the difference in speed didn't pass 7fps, I'm also thinking that the quantity of cars gives a higher framerate drop than the actual total polycount of them for some reason, my guess is that the amount of separate objects is what causes the drop.
I also heard that AC will be lighter than pCARS(I wouldn't doubt this one since pCARS is by far the heaviest game I have here), actually I think AC will be a lot lighter than pCARS.
I guess it wouldn't make much difference in the end if you use a very highpoly model with as few separate parts as possible for the player and LODs in the same level as the other cars in AC for the AI.
Also, I understand where you come from, I too can't hold myself while modeling, I always try to make things as smoothly and detailed as possible. the original mesh for my Prowler had 30k polygons, it has around 9k now after some heavy optimization, but even so I don't want to give up on the smoothness, I'm still smoothing some parts here and there. hehe
Oh and that pic you posted exemplifies what I said earlier, in that screenshot it doesn't matter if the car has 280k polygons, it looks just like any 20k~30k polygons car there.