If you want to save a lot of weight...
Buy a barrel-style bag, like maybe a Western Mountaineering Aspen MF (25F, 1lb15oz).
Hit Seattle Fabrics or some other fabric place and buy matching zippers and enough lightweight polypropylne-type material to make a bottom sheet. That will probably end up weighing six or eight ounces, a lot of that in the zipper. Get the lightest fabric of that type you can find. If you can find the equivalent of "silkweight" polypro (or heck, even silk) you'd probably be happiest (right now I have this vision of enormous silk tablecloths I saw in a street market in Thailand, they were selling for maybe $25 and were easily large enough -- though here in the states you'd probably spend more for that piece of silk than for the sleepingbag).
Have a matching bag that you can zip together for colder sections of the hike, and have someone back home who can mail that bag to you on colder sections of the trail. Probably that sleeping bag should be a mummy-style bag that is rated a bit colder than the top bag you'd normally use, but still have a compatible zipper. You can do this with most WM bags.
WM sells a "summer coupler" that you can use. but it is heavy (1lb 4oz) and kind of spendy. Better to try to make your own. I also dislike the "summer coupler" because it appears to be made of cotton.
I and a lady friend did quite a bit of backpacking with the above setup, and it worked okay most of the time. There were three problems:
Male and Female thermostats are set a bit differently. This is a lot more noticable in colder air temperatures than it is inside at home.
The bottom sheet doesn't compress very well, and ended up being nearly as large as the sleepingbag (the above mentioned Aspen MF).
You had a big hole at the top and lost a lot of heat through there.