There is a good podcast on BIAB on the Beersmith website with Pat Hollingdale, who is one of the early adopters/contributors to the BIAB method.
http://beersmith.com/blog/2011/02/24/brew-in-a-bag-biab-down-under-beersmith-podcast-10/
On this topic of mash thickness he says the difference for BIAB vs traditional 3 vessels brewing is the sparging. In 3 vessel you sparge (dump hot water) over the grains to rinse out the sugars. With BIAB you have all you water in the mash already which Pat calls a "passive sparge". Basically the thinner mash allows the sugars to be rinse away a little more easily then a thick mash. It is worth a listen.
For what it's worth, it seems most BIABers get about 70% efficiency, which is what you will find most AG recipes are based on. Don't let the efficiency number become a pissing match or some elusive pot of gold. 70% is good enough for home brewing and a full volume BIAB mash will get you there.
Chris