putting the weight vector/dive angle to the side for a bit, I think what is missing here in the discussion is the drag vector. Now, beyond peak/vmax. what is happening is the drag vector is slowly overwhelming the thrust of the propeller as it gets more and more inefficient until efficiency is at zero and advance ratio reaches some high number at some given high TAS. So, it is negative excess thrust between those two vectors because drag force is becoming greater than the prop thrust vector (still ignoring the weight vector for now). So, the question is how is the interplay between these two vectors modeled in IL-2. Maybe it is already accounted for in the modeling of the drag force? Like an effectiveness factor or something that is attached to the drag coefficient as the thrust begins to degrade beyond Vmax? I really have no idea, only speculate.
edit:
of course, the only way to get beyond vmax is to dive, and its the sum of all forces...etc. so this interplay between opposing thrust and drag vectors gets masked over by weight/dive vector. am i on the right track?