As nice as the heater body suits and iwom's look I can't imagine using them with a saddle either. I'd be interested to hear if anyone has used one successfully.
I'm sure extreme cold is different for different areas of the county, but for me it is between 0-20 F. I have used my system to hunt numerous times in the past couple years in these temps and it works for me.
I have the cabelas extreme stand hunter coveralls. I think they are awesome. I pack them in and put them on at the bottom of my tree. Under that I wear 1 light layer of scentlok, a fleece vest, a layer of cabelas polarfleece, a layer of first lite merino wool and an under armour base layer that wicks. For my feet i have l.l. bean pac boots and I wear a moisture wicking sock under a pair of good merino wool socks. Depending how cold it is i put either 1 set of toe warmers on or if its really cold i sandwich my toes between them. I always use a hand muff if it is cold at all because i wear light gloves, but the stand hunter coveralls come with one that attaches to it. I throw 2 packs of handwarmers in there and I'm good to go. Again depending on how cold it is i wear something over my face, either just a lighter camo cover or if its really cold a fleece baklava. On my head either a regular hat with the hood pulled over or a wool hat.
I used to freeze my butt off out there all the time, but this system has allowed me to be able to stay on stand for full days in the cold. With the coveralls I always draw my bow when I get in the tree to make sure I don't hit them when I draw back, sometimes I have to adjust or strap it out of the way.