I've done BIAB using my Dark Star with an 8 gallon kettle. Here's what I have found. When I add the grain, the closer the water level is to the top of the kettle, the easier it is to keep a temp. My last batch was a 3 gallon BIAB so the kettle was not nearly full during mash. I had a heck of a time keeping the temp. But I don't think it had anything to do with the Dark Star. I think it is just easier to keep mash temps in a cooler or something insulated. Now, I live in Southern California, so I usually brew in 60 to 70 degree weather (yes, even in winter, sorry). So I can only imagine how hard it is to keep a kettle at a steady temperature when it is actually cold outside. Your best bet is probably to get something to insulate the kettle with.