I currently play a GR 65 (.650, number 3 backbore) C* mpc. Prior to starting the course it worked really well for me until I moved up to 1st chair in my community bands. Along with the extend ranges of the 1st parts and learning a new way of playing via the WindWorks course, my jaws and cheeks were getting very sore, to the point I would have to stop playing for a few seconds, especially when playing in the upper range. So I downsized to a .640 mpc with slighly tigher backbore (number 2 backbore). This seems to have helped, allowing me to playing the upper register easier with less soreness.