I will be thru-hiking the John Muir Trail this summer and would like your opinion on sleeping bag. I currently own a synthetic, zero-degree bag that is an unappreciated 4 plus pounds. I know this is too heavy (as I thru-hiked the Colorado Trail with it). My real question: can I get away with a 30-degree down bag, or should I look more at a 15- to 20-degree bag? I tend to be a somewhat cold sleeper. I am 6'2", 190 pounds. I know the temperature ratings are guestimates at best, but your input would help.
Well, if it were me, I'd go with a 30-degree bag. I really doubt you'll see many temps much lower than that, and you always can go to bed with extra clothing on—just wear your fleece jacket to bed.
As to which bag, three come to mind. For several years my go-to bag has been the Marmot Arroyo ($269), now in its second or third iteration. A great bag, with a 30-degree rating, 800-fill down, and light nylon shell with a DWR treatment to shed light moisture. And weighs 1 pound 11 ounces in regular; you'll need a long, so add a few ounces for that. Packs down to the size of a softball, and is very comfortable.
I also really like Mont-Bell's U.L. Super Spiral Down Hugger #3 ($305 in long). Weird name, great bag. Rated to 30 F, and has a stretchy shell that "hugs" you very lightly. That's a good thing, because it eliminates air gaps inside the bag and reduces "bellows effect," the tendency for a bag to draw cold air into it as you move around. Very light as well. Only 1 pounds 8 ounces in the long.
Lastly, take a look at the Feathered Friends' Osprey ($344 in long). This is a really high-end bag—almost a custom bag (in fact, you can customize it by having it over-filled a little). Rated to 30 degrees, 1 pound 11 ounces, super high-quality. Cut just a bit wider than some bags, for a bit more comfort.
Hope you have a great trip! That's beautiful country.