I have Sidi Blaze model shoes but I guess I have a little more wriggle room than you do.

At around 50o or so I start wearing heavier wool socks. Smartwool.

At 45 I add toe covers - Callientoes. You can get them here at TE. They are wonderful!

WHen it goes to the 30s I add neoprene shoe covers. Mine are made by Terry and worth every penny.

If it gets any colder than that I'll be snug as a bug in a rug sitting in front of the fire with a good book.