If you are using it in a swag, forget it, get a sheet and a doona or a couple of woolen blankets. This way you can control your temp by opening up the sawg and blankets and you can dispense with blankets or doona in summer. I have slept many nights in a swag as it was my choice of accomodation for work for three years and I never used a sleeping bag, always sheets and blankets.
I have a fairydown sleeping bag that I bought when I lived in Tasmania. 600grams of down from memory and gortex outer shell. I have never camped anywhere where it has been cold enough to sleep in it, I just wake up after an hour half cooked and soaked in sweat. I have tried it open as a doona a few times and it is still to hot. As it packs down to next to nothing it is usually throw in as backup as you can be assured that if it gets to cold for everything else we camp with, it will do the job.
Short answer, for a swag, forget the sleeping bag and fancy swag bags and buy a cotton sheet set and blankets.