I can't put a number on it, but I can tell you the first thing I noticed
when I started shifting manually. Without having to watch the tach, you can
judge by the sound of your motor (when not in traffic) how fast you're going
at what point in the turns. The best way through a turn may be high 2nd
gear, but the auto-shift may leave you in third. It's much easier to judge
your speed if you know for sure what gear you're in, and you know it will
stay there. Also, at some tracks you can use engine braking to advantage,
that is, you can wait a little longer to brake, then downshift a little
early and let the engine help slow you down.