This will only be a partial answer to your queries.
"Does this reduce the temperature rating, or is that only if you can't sit still and let air in"?
Once you lie down inside of your bag the insulation that you are lying down on is flattened and reduced in efficiency to effectively "0". That is why we use sleeping pads and refer to their R value.
If you let air in, yes you will let heat out. I use a quilt and you do have to adjust the position of the quilt if you roll over on your side. A properly sized and fitted quilt with the footbox cinched or sewn closed and the top snapped and cinched about the shoulders and neck will keep the edges of the quilt under your body.
"What do people who have converted their bags think of the new product"? "Worth the effort"?
I haven't converted a bag to a quilt. I have used bags and I have made and now use quilts. Yes, it was worth the effort. Yes, you will save weight. Yes, you will have to adjust your methods somewhat.
I have no experience with down or baffles. The article you reference is the best that I have seen on this forum for converting a bag into quilt.
Good luck with your decision and your project if you choose to move forward.