S!
I am a n00b with the FMB, but can try
It seems the lowest FPS comes when you go below the clouds, not so much above them. I had no stutter, it was quite smooth except you could feel the low FPS.
Will run more tests to get more consistent results. The Black Death is not IMHO a good measure of performance as it has graphical glitches I never see in game, like flickering explosions, the strange smoke "streak" from the fallen Me110C, flickering cloud shadows etc. My guess is that as it is made with another version of the game it does not work proper on newre ones, same could be seen on IL-2 BD track.