I use a 15.5 G keggle for 10 gallon batches using a wilser BIAB and a hoist. My last batch probably had as much grain, 32 lbs, as the most I've ever used as mash water was spilling out of the top of the keggle. I had actually planned to use 2 more lbs of grain but there was no way they were going into the keggle without a corresponding amount of overspill.
The mash consistency was very thick but I mashed for 2 hours, sparged between 1 and 2 gallons with room temp water after the mash, and ended up with 76% mash efficiency.