I have two winters in Boston under my belt with my S85. I've taken her on several frigid trips (below 10F) to places like Clinton NJ, Saratoga Springs NY and North Conway NH. In the extreme cold (and sometimes with headwinds) at modest highway speeds I have seen sustained consumption on the order of 375Wh/mi so about a 25% reduction from "rated range", my summer driving is more in the 325Wh/mi. What gets us here is both cold temperatures and also hills (folks in flatter places report sub 300Wh/mi regularly, I find that possible on secondary roads in the summer here but no way in the winter on highways).
I did go with Tesla's "Winter Wheel and Tire Package" - these Winter Pirelli's are excellent cold weather tires, but I have to say they are not great in the snow/ice compared to other true snow tires. They are a nice compromise for most situations, but I did drive in a couple of our many big winter storms and I have a very steep 400 foot long driveway. With an inch of snow and the weight of the car, I couldn't make the hill. But, I am confident that I could have with different tires - There are several forum posts on winter tires and the Nokkian Hakkas seem to get a lot of praise, I would switch to those, but figure it hasn't been too bad and I'll live with the Pirellis for at least another season or two since they are otherwise perfectly adequate, especially on flatter roads.
I also bought Tesla's winter chain package for "just in case" - and I ended up using them when I went to work during a blizzard (not realizing how bad it was going to be), bailed at noon to get home, but none of the roads were plowed and I knew I wouldn't be able to manage the hills safely, so I threw on the chains and lumbered home and up the driveway, no problem. Glad I had them, and glad I practiced installing them before I needed them.
A final note, when I started there was no supercharger build out available for most of my trips. But since last Thanksgiving, we now have West Hartford, Auburn, Springfield, Hooksett, Kingston, Bourne, and others that have come in handy, and Albany in particular has been key for my trips to Saratoga. This really makes life a lot easier and planning less of a hassle (though I enjoyed plotting out my course and living on the edge a little).
Cheers from Concord, MA and go ahead and pull the trigger. It's an awesome machine, welcome soon to the club.