My bag is rated for 20degrees. I did a forum search for recommendations on temp rating of bag for the transAm and that seemed to be the consensus so I ordered that rating
A few factors come into play on that choice:
1. Bags are very often not rated honestly. When comparing, using the EN ratings is the way to go if the manufacturer gives them.
2. Users vary widely on how warm they sleep.
3. Supplementing with clothing can allow for comfort in lower temperatures.
4. Being in a tent or a bivy make the bag warmer.
I will say that for me your bag would be way more bag than I'd want on the TA. I did the TA with a real cheap bag nominally rated at 32 F and was fine.
I did the ST with a good down 40 F (EN rating) bag (Mountain Hardwear Phantom 45) and was fine down to the 18 F low of the trip with tights, a warm pile sweater, and thick socks inside my bivy. My bag weighs 1 pound 1 ounce and packs a little bigger than a large water bottle. I love that bag and found it well worth the $280 I paid for it. I apparently put out heat like a furnace though.