I have Dubarry knockoffs (Dublin boots), and if you put Welly socks in them they are very warm.
I teach all winter long in an unheated arena and I have Raynauds. The biggest trick to warm feet is keeping them dry! For the really cold days I will put boot warmers in my boots too.
Not a huge fan of Mountain Horse - I find they don't last very well and are not very waterproof. My Dublins lasted 7 months of continual wear, and I've just now scrubbed clean and re-waterproofed them.
They say money doesn't buy happiness -- well happiness doesn't buy horses!