I use a WSM 18 completely stock.
I've never had problems with maintaining temps for a sustained period of time in the cold.
My experience may be different from others. I have not ever used water in mine. Instead I run my water pan half full with pumice stones, with foil wrapped over the top of the bowl and a depression in the middle for the drippings to collect. Makes for easy cleanup too.
I have only used Stubb's or weber briquettes with oak or hickory chunks. I have had several cooks that my coals have lasted up to 16hrs.
However I do not remember the wind conditions from those cooks. Maybe the wind was calm.
I have thought about cutting a 55 gallon drum for a windshield though. I think that the wind affects it more than the outside temperature does.
What fuel do you plan on using?
This is a pretty typical run from a fully packed ring of Stubb's and oak.