In the 15 or so BIAB batches I've done, I have ranged between 78 and 85% efficiency, much better than my batch sparge setup. I don't bother with a sparge or any of that, pull the bag, loop the drawstring on the hook I installed above my kettle and fire up the burner. While I'm measuring my hops the wort is coming to a boil.
To the OP, if you're concerned about lower efficiency, you should try brewing a batch you're familiar with but do it BIAB style an compare your numbers. I think you'll be surprised.