Flying at the thin end of the envelope has hidden risks that the certification process does not allow for.
For instance, when I first flew the SJU-GTK-MIA route I was mystified that several times the autothrottle seemed to be having a problem with speed stability. It would sometimes spend several minutes at the high or low end of the normal range just to stay close to target speed.
What I finally deduced was that when the upper winds were strong and out of the West, often there were standing waves way downwind of Hispaniola, and I had been surfing up and down those waves, which were almost parallel to the route.
Now think of an aircraft at max alt, with very little thrust margin left, and throw that type of effect in, and you may have a problem.
At times those standing waves seemed to have the equivalent performance effect of 200/300' per min, but that was subjective judgement.