My first thought was: he is trying to accelerate from 20 mph in sixth gear but then you explained that you are an experienced manual driver and the right gear is not the problem. One of the things I like about my car is how smooth it is accelerating from low rpms in just about any gear due to all the torque available. I can only tell you that I have never experienced what you are describing and you might have a problem with your car. Now, before you take it in to the dealer to have it "fixed" make sure that you try any other possibility for the problem - like a need for a setting on iDrive or something that is not set correctly, e.g. settings in Sport Mode that quicken the throttle action. Do you have adaptive drive with normal, sport, sport +, etc.? If you do, try driving in Normal mode and see how it feels compared to Sport and Sport +.
My advice is: don't take it in to the dealer to have it fixed because they will not only not fix it but you will develop problems that you never had before. If it bothers you that much and you cannot adjust to it then you will have no choice, and I wish you all the luck because you will need it.
