I've been using 1000 gram thinsulate rubber boots for years in "extreme" cold temperatures, sometimes as cold as -20 degrees.
My feet usually stay warm enough to handle several hours of hunting but my face, ears and nose are usually what fail first.
I try not to over-dress my feet, I usually wear lightweight socks in rubber boots but I carry extra socks just in case.
I see a lot of guys over-dress their feet with thick woolrich socks but too much sock causes sweating and sweating leads to cold feet.
Another tip that helps when cold feet becomes an issue in extreme cold temps is to wear a pair of womans knee high (nylon socks)
"UNDER" your regular socks, doesnt sound very manly but while the other guys are huddled around the woodstove you're still out stalking the Bucks!