I've done BIAB, cooler but then I saw the light and made a keg mash tun.
Heat the water in the keg mash tun to dough-in temp. Mix in the grain, stabilize temp and let it sit. Direct fire to get up to sparge temp, drain and sparge. DONE!!!
I will admit to using the BIAB bags for other aspects now. Such as for the luscious wort to flow through (from the mash tun) so that I don't need to vorlauf. A few ounces of rice hulls is sound insurance against a stuck sparge, when using a Jaybird/NorCal false bottom too.
As for efficiency... As long as your consistent for different grain bill sizes, who really cares? I range from about 72-78% currently, depending on the amount I mash (full keg is ~72%). With buying grain in bulk, I have enough on hand to brew my recipes without issue. Personally, I'd rather sacrifice a few efficiency points if it means I'll avoid the negative aspects of too high an efficiency (tannins, etc.).
BTW, another way to have better efficiency control is to mill your own grain. Use a solid mill, and you eliminate the HBS doing a good enough job of it for you. Plus, milling the grain shortly before mashing it adds another level of freshness to the ingredients (at least to me it does). Besides, running my mill sounds so damned cool. Well, evil actually, since it IS a Monster...