Could the variance in speed be due to the fuel? You say you are at 100% fuel and no burn. It seems to me IRL it would take a bit of fuel to get to 16,500 feet. What was the fuel used in the original bug report?
I am just a tad above we-todd-did about these things, but it seems to me even a slightly heavier plane would need different trim settings at alt to provide the lift, thus possibly increase drag, etc.