Batch size: I just participated in a partigyle where I produced one and a half beer in my BIAB mash tun and a fellow sparger produced one and a half beers in his. The two halfs were mixed together so we had 3 beers from two tuns. Each tun had 24 pounds of grains. I did a Tripel and half of a Dubbel. So yes, high gravity beers are totally achievable.

I even got a bonus sour beer out of my mash by tossing the bag in a cooler with 160F water for 24 hours before boiling.

Efficiency: I get better than 70% efficiency. Double crush the grains, do a mash out, and squeeze the heck out of that bag. Want to really boost efficiency, then do a 90 minute mash with the kettle wrapped in an old comforter or sleeping bag.

Also, it is much easier to just use a cookie rack in the bottom of the kettle and put the bag in before heating the water. Use office binder clips to hold it to the sides. Make a bag out of swiss voille from the fabric store, stitch a high strength seam and you’ll be fine with almost any amount of grains.

I don’t have enough BIABs under my belt to compare the body or dextrines. But I have done pale ales and porters. Both had the correct body for the style.