R13 is meant for 4 inch studs, if you can stuff in R19 more power to ya. Most houses in my area only have R13 and it gets 0-100 F here.

If you really want to go all out I think you have the right idea hitting all your corners with Great Stuff insulation foam. I would worry more about your ceiling and floor then the walls obviously. You'll lose way more heat out the top and bottom then the sides.
Go thicker on the ceiling, R25 if you can make it fit. Its all going to depend on the thickness of your studs and trusses.

If we could see some pics of the shed it would be alot easier to make suggestions...


Another thought, after the R19 on your walls put up some deafening board and then sheet rock. It is cheap stuff and helps insulate.

I work at The Home Depot, "You can do it, we can help". lol