I have the Ice Breaker Boot Blankets, which are the big fat puffy ones. They will fit over my 600 gram lacross boots and will defintely keep you warm. I tested them the first time by only wearing one, and it made a huge difference that was obvious within an hour. The uncovered foot got cold quickly while the other stayed toasty all morning. They are a big pain to carry due to their size though.
If I can hijack your thread a bit I have an additional question. I also tried the Arctic shield brand a couple years ago because they are so much more portable. They were warm, not as warm as the Ice Breakers but the portability made up for that and were warm enough 90% of the time. I hated the Velcro closures though and the design was hard to slide on your boots. The new ones shown in Cabelas, etc, show a drawstring around the top and a zipper in the back like the Ice Breakers. Has anybody tried the new AS style and if so are there any drawbacks?