Grain bill and everything for BIAB is just like "regular" all-grain brewing. Personally I find that with a good sparge, I can get efficiency in the upper 80s just like using a cooler -- there's no difference for me and my techniques between the two. I usually sparge by setting the grain bag in a large colander and slowly pouring 190 F water through the grain bag as something very similar if not identical to a fly sparge. Other times I just dunk the grain bag in 170 F water for a quick rinse. The reason I prefer the colander method is to avoid getting too many grain particles in the wort. I guess if I had a very fine mesh colander or grain bag this would not be as much of a concern, but on the other hand you can certainly still get a "stuck mash" if the grain bag is too fine. It's a good method to play around with, including different mesh and sizes of the bags, etc. But yeah... bottom line is, if you want to get high efficiency with BIAB, it is certainly possible. I have gotten efficiency as high as 92% with a really fine crush and good sparge, and you can too with experience. In fact I've dialed down the crush on my grains because I fear that efficiency that high can result in lackluster malt flavor due to so little malt required to hit your OG. So I purposely try to hit about 85% efficiency, and have been successful doing so on most brews. Or occasionally for very malty or low gravity beers, I just don't sparge and then efficiency will fall to low 70s, which is fine too. Whatever you want to do, it all works.