First, congrats on your bird!
I don't think you'll be happy with your current tarp. Blackbird just takes up some real estate. You mentioned 'cold' being one of your priorities, so I'd stay away from any diamond shaped tarp; no real side protection from the wind. The Blackcat is a great cost option, but it does take either a little skill or a butt-load of patience and will have the same cons as the MacCats, or any other hex. Warbonnetguy makes a couple of tarps that would meet your needs too. He has a smaller version of the Big Mamajamma, but I don't see it on his website. Again, hex tarps.
For someone concerned about the cold, their budget, and weight I'd pretty much have to steer you towards the Speer Winter Tarp
. It provides in all three of those categories. It provides a large footprint and can be sealed-up reasonably well, it is a bargain at just over $100, and all that for 16 oz. Biggest "con" based on your hex concerns is the need for a couple more stakes.
It meets all your criteria with one negative. In my experience, that's about as close as you can ever hope to get.