Double D,it is all the same things that you hear over and over again.Make sure you have plenty of room in the boots for heavy socks and so your feet are not constricted.I would go at least a full size larger than your boot size.Heat packs in the boots or the use of ice breaker boot blankets will keep your feet warm.The advice about keeping your head warm is right on the money.Personally I rarely hunt in boots with more than 600 grams of thinsulate because I am typically walking quite far to get to where I will sit,I rely on loose boots wool socks and heat packs or the boot blankets.
I have had very good experience with Schnee's boots,they make a cold weather pac boot.I have heard a lot about Lacrosse's cold weather pac boots,but I have not had first hand experience.
Hope you find something that does the job!