Any insulation on the bottom is going to be compressed and worthless no matter what bag. Thats one of the reasons for a pad - any pad. All BA did was integrate the pad into the bag via a sleeve.
I use a BA, along with several other bags and the BA works best. I have a 35 degree rated bag and with the air core insulated pad was able to sleep comfortably at 28 degrees inside the tent. But I could feel the cold creeping in and don't think it would be comfy much colder. I pack along a aux thermal liner though for times like that, just didn't get around to it. i like the 35 degree bag because its light, packs to nothing and works to a bit past the rating.
I have no problem sitting up in my bag with either an insulated air core or uninsulated air core mattress. Not sure why it wold be a problem.