Yes, there are windows darker than those who ship with S2 and Viewer.
But, I have not seen any modifications whose windows are only have as "opaque" as the "stock" windows on the cars.

At least, if I want an FPS drop I'd like to see a bit more <in a non sick manner>!

As someone who's native language appears to be English, your writing is bad. It took me a while to even grasp your topic.

AFAIK, there are no variations of glass opacity possible but I might be wrong.
I have yet to notice any difference so it hasn't caught my eye.

Someone correct me if I'm wrong, please.

LFS S1 had darker windows IIRC. Currently, what you see is what you get and there is no way to change the way windows look.

That's simply not true. Back when we were at RSC, I made a bunch of different opacity/hue window textures for LFS. Go search there.

Quote from Batterypark :

That's simply not true. Back when we were at RSC, I made a bunch of different opacity/hue window textures for LFS. Go search there.

Yeah, you're right. You can change the way they look on the outside of the car. The inside view technically has no windows though. Also, other people online won't see anything different unless they also have the changes on that car as well.

LINK.