Ah heck, in that case you could probably run a cooler bag; 15 vs 0. There so much room in those things you could wear all your puffy clothes and keep your boots on!
I am a cheap ass, or poor ass, so I lug in a MSS that I got for $139 new. Usually just use the patrol bag. In summer I use a USMC poncho liner with zipper. When I get some money, I think that I'll try an enlightened quilt.
Keep in mind as the temp drops you need to increase the insulating factor of your ground pad too...what's under you is equally as important as what's above you.....pro lite gear had an awesome sale on sleeping bags in sept..,,picked up a WM kodiak less than 500