I absolutely agree with this. A 500 DO to replace the 400 DO that is a stop faster than the 100-500 zoom would be a tempting lens and I think it actually has a chance of appearing at some point. (Basing that on the fact that Canon found room in its lineup for four 400mm EF lenses and DO adds an additional differentiator)As a scientist I greatly appreciate you performing these measurements. I found them very informative. One benefit of a 500 mm f4.5/5 DO lens would be the lighter weight and knowledge that I can place a 1.4x TC on it and have a 700 mm f6.3/7.1 which is still very practical. It would also be smaller and easier to travel with as well than the 200-500 mm f4 zoom. As I mentioned before, two different lenses that IMO could be complementary.