Interestingly enough I just did another test and now I saw a major drop when switching the external lights on in the NGX standing at gate 6 at Orbx ESSA. The only thing I can think of that changed since I did the last test is I've applied the last fix for Orbx ESSA. However it doesn't really matter because the FPS is still very acceptable and won't cause any issues from a performance point of view.

Here's my results:

With 4xMSAA it dropped from 50 FPS -> 30 FPS when I switched all external lights on
With 4xSSAA it dropped from 26 FPS -> 12 FPS when I switched all external lights on

However since I'm running at 4K I don't need SSAA meaning I get 30 FPS which again is totally fine. This is with FPS set to unlimited and using the settings found in the screenshots above.

I have triple monitor setup and with NGX I have about about 35-45 fps with with 8xSSAA. If I turn DL to ON fps drop down 9-12 at night. I use 2xSSAA for night flying. If I switch to a single monitor I fps looks a bit better. 11-15 with DL. 

Let's hope this will be improved in 4.1