It's a good site isn't it?
I've always felt like a large handlebar bag slows me down massively (I'm guessing this is made worse because I'm fairly light so don't have much momentum).
Jan Haine suggests that a bar bag acts as a fairing although he's talking about the rando box type rather than the longer rolls like a sweetroll etc. I'm not sure, Jan is a man of proper scientific methods though I don't know if this came from on-road testing or wind tunnel data. Wouldn't care much tbh : ) If I did I'd use tri-bars .. and one aspect in isolation is different to how it works on the road. ie, yes very supple Compass/Panaracer tyres do roll very efficiently, but I lose far more time than they gain me by fixing the 2 flats a day I've had with them on a 4 day trip.
I can see some logic in saying that a bar roll of the right proportions might direct air more cleanly over the legs and body anyway. Yet the fast road event riders (Like Kristof Allegaert) use a seat pack, frame bag but no bar roll. Fair to say that I've never seen a rider with 4 panniers going very fast, so must be true, right?