Whats apparent from this thread is that we each react to temperature a bit differently. I tend to think you will be OK with a prim-aloft 60 grams parka (xenon) or mb thermawrap parka and a summerlite, perhaps with some additional insulation like a vest or fleece. I have not been on a thru, but I've sectioned about 500 miles of AT during all four seasons. I generally don't pack to be comfortable in the coldest projected temperatures. Its not fun to find yourself shivering at 5am, but there are easy ways to deal with that. Do a bunch of sit-ups and crunches and eat some almonds or chocolate to ramp up your metabolism. Boil up some water, and put it in your nalgene etc. Or just roll out of your bag and start hiking.
I'd recommend the xenon for the same reasons that Matt recommends the thermawrap. Its just more versatile, especially if you plan on hiking for most of the day. In my experience, a down garment stays in the pack all day with my down bag until I reach camp, which sort of limits its use, since I don't spend that much time in camp. The flash is no doubt warmer than the xenon, and if you have the money to spend, I doubt you'd regret it for a second. Its an awesome jacket and I miss having mine already. However, if you can get the xenon on sale ( I got mine for $110) , than I don't think you can get a warmer/lighter synthetic jacket. Like Matt, I've worn mine in a downpour, only to dry it out with my own body heat in about an hour. The shell is super wind proof, and quite water resistant. I've owned a thermawrap as well and that kept me warm to about 40, and the xenon feels warm closer to freezing. I think its something to do with the fact that its not sewn threw. Just seems to trap heat better.
The good thing is that you live in NC, so you should get the chance to test out your system for at least a couple of weekends before you hit the trail in march. I'd go out with some extra insulation (doesn't matter if its UL) with your current bag and the jacket you intend on using. Check the temps and see whats comfortable. Beyond that, I wouldn't worry about it too much. Keep your eye out on gear swap as well. You may be able to get a lightweight used synthetic and a warmer down (for the first couple weeks) for about the price of one garment. I'm selling a thermawrap parka in size L if you're interested.