Are these light weight and flexible? and...
Are these light weight and flexible? and if I have K2 transit boots size 11 should I get mediums or Large?
The mission bindings are lightweight and have a good flexible feel. I use the 2010 mission bindings on my 2010 burton hero and it's a great combo. I have a large size bindings and a size 13 boot, and there's still plenty of room so you should be fine with a medium.
Shred is correct, these are great and flexible but not too soft. The medium will be too small, run the large.