trick is eliminate circulation
It seems a true peapod is tighter around the hammock than a sleeping bag with a foot opening is. Is the extra space between the bottom of the hammock and the sleeping bag a problem?[/QUOTE]
The key to warmth is to eliminate air circulation that could result in air flowing into and out of the underside of your peapod bag, taking away your body heat. Arguably, that extra air space that you report could be a valuable addition to your warmth, as long as the air in that gap is trapped and cannot flow in and out.
I use a 40 degree Kelty Lightyear down bag in summer as a peapod, and a 15 degree Sierra Designs Nitro 15 degree down bag in winter. Both have zip-open foot boxes. The mummy shape, for my 5 ft 10, 150 pound frame, keeps the bag relatively snug at the hood end, when zipped all the way up around the hammock. In early March I stayed toasty at 5 degrees (wearing merino wool top/bottom, down jacket, merino balaclava and fleece beanie).
The key is minimizing air flow out the foot box and out the top. At the footbox, when it's cold, I stuff a plastic grocery bag at the bottom stick my feet in the bag. The bag crumples around and forms a pretty effective seal against air coming in the footbox. With the bag fully zipped, the collar/hood area stays close enough to the hammock bottom to maintain continuous touch with the hammock fabric. Not tight. Just touching. System works great for my in my Grand Trunk Nano 7. I am very comfortable in this small hammock. And for winter camping, can you imagine a lighter weight system, with less fiddle factor? I love it. But according to many others who have tried it, it's not for everybody. Your results may vary.