I love the Sitka gear, especially that Celcius vest. That's the one piece from Sitka I'm hoping to get this year.
That said, I don't think the Sitka gear was designed for the grueling sits in fridged weather on the stand. The system is more designed for maintaining your core while on the move. You could use it for stand sitting, I just don't know how effective it would be when the temps really drop down and you're not moving.
My recommendation, even though I don't own one - yet - would be for a Heater Bodysuit. It looks like a big camo insulated sleeping bag that's windproof, waterproof, with separate feet. It's warm enough that you can wear the lighter cloths on the way out the stand and not build up a horrible sweat, and still be warm and toasty after you arrive.
I know a bunch of guys that swear by them, so that'd be my choice.