I'm a bit on the fence when it comes to this. When it comes to parking lots, I understand. For this issue, you could consider making a /report and see what an admin thinks about the issue. On the other hand, a bunch of cars is pretty normal for a dealership to have. It is one of the few things in GTA where your business is visible beyond what is already there from the base game.
As for performance issues, leave this up to the admins. If they deem that there are too many cars in a location, they can despawn them. I can't say I ever ran into any of these issues, even when Autopia had 80 cars spawned. The game is years old by now, and anyone with a reasonably modern PC should be able to handle this game without trouble.
What really annoys me most from these screenshots is the /carsign draw distance. Those signs should really be visible only when you're standing directly next to the car in question. It clutters up the screen way too much to the point where you can't take proper screenshots (because the signs can't be hidden) or you can't properly read your chat.