Yep, the double sparge (a single is fine too) usually does give quite decent efficiency, the way I look at it is that sparging usually yields around 6-8L of about 1/2 the SG of the mash, equating to nearly the same volume of beer. You can add more grain to compensate, however because the kettle volume is limiting (mashing with the 'tun' full to the brim), the liquor yield goes down and sometimes without the commensurate increase in SG, so that can be counterproductive. I haven't really explored these sorts of limits seriously (on the to- do list!), but I think largely by chance the method hit on a fairly sweet spot from the word go for 5%ABV.
Another aspect to sparge is that, while we're dealing with a concentrated wort, it distributes the total sugar load throughout a greater volume, so fixed losses into the fermenter are lower if all else remains the same. You could do MaxiBIAB with just water plain additions for the boil however sparging with it does something much, much more useful.
With the brim- full method, nowadays I usually prepare about 3/4 of the kettle volume of strike water to an overheated temperature, place the kettle and insulation where it will be mashing and allow things to warm up for a few minutes, then reserve a few liters in a jug, add the grain, stir well and then measure the mash temperature- if it is low, add some boiling, OK then add the reserved, high, add some cool. That way you don't need to calculate that much beforehand, get precisely the mash temperature you need while also saving a heap of pain and heartache agonizing over the volume and temperature numbers- that's perhaps the number 1 challenge for novice AGers.
I like to use flowers as much as I can, however there's not a huge range here in Aus due to strict quarantine restrictions, they're only available from New Zealand and of limited varieties so often I settle for European/ UK/ USA plugs (plugs are usually just compressed flowers- not always though!
). If you do follow the stock MaxiBIAB method it uses a colander or a big sieve for straining the cool wort into the fermenter, when used like this the loose flowers do trap an enormous amount of break material* when just poured straight through whereas pelletised hops debris doesn't really do much except block every filter up. So, its flowers or plugs for me!
* BTW, break material isn't really the big bad bogeyman that it is often made out to be, you'll probably still do quite OK even if you end up with all of it in the fermenter, but flowers (or most plugs) will do an acceptable job in reducing it. And before anyone goes on the tilt over it, I will hasten to add that break reduction can be considered an incremental improvement that can lift the quality of your beer.
Hope that helps!
