Thanks for your response, I really appreciate it!
If you're certain mediums will be fine then I will get a pair now on discount.
I have read other people say that their size 10's are really tight with mediums and they wished they'd bought larges though.
I think it depends on your boot shape. I wear size 7 malamutes, and the heelcup on the mediums puts some savage pressure on my heels. I can't imagine fitting a foot three sizes bigger in there with the same boot.
As always with bindings, you'll never know how well it fits until you strap in.