Yeah, the Appalachians in October or even late Sept can get chilly a night. I wouldn't bring a 55F rated bag any month of the year in Georgia.
In my personal opinion, based on owning a lot of sleeping bags over the years ranging from cheap synthetics to "good" synthetics through cheap down to the very best down bags, the sweet spot for cost vs quality right now is a quilt from Enlightened Equipment. A 30F rated quilt is about $250 -- expensive, yes, but compared to $450 or more for a similarly made bag from Western Mountaineering, it's a bargain.
As you predicted (and proved), switching to a light pack while using other "traditional" gear doesn't work very well. It's a process