There is a fundamental difference between the Exhaust brake you guys use in Europe and our Jake or Jacobs- Brake. The European Exhaust brake is a flap mounted in the exhaust pipe right after the manifold closing most of the exhaust pipe off to create back pressure that will slow the vehicle down, the Jake brake on the other hand works by opening the exhaust valve when the Piston reaches TDC thus releasing the compressed air and slowing the vehicle down and makes for this typical brrrrrrrr sound when activated.
And yes it would be nice to see an actual jake brake simulated in ATS, but I wouldn't hold my breath