I love BIAB! One thing I have learned is to use Brewing software to get your efficiency, grain absorption, and temp calc right. Once you get that dialed in, it really comes out great! My kettle rarely can hold all my water/grain at one time. So I always calculate for 8 gallons of water, then add the rest as it is getting ready to boil. You don't "have" to use all the water up front.. This allows me to really hit the OG everytime and not spill over. I also always do a Mash out (five points OG) and a light bag squeze and drain.
~ BIAB : All Grain Made Easy ; Mash, Sparge, Boil all in the same Kettle ~ all you need is a bag and a hook!