I'm a diehard BIABer. I built my brew cart, with 2 burnners and 2 16gal bayou classic 1064 kettles, around the process I thought worked best for me. It still works great. I'm still threatening to convert to electric brewing and if/when I do I'll likely keep all the same gear.
One kettle is the MT/BK. The second kettle is the HLT.
Typical 11 gal pils batch I'll heat 9 gals strike in the MT/BK. Why 9 gals? Because I can use a false bottom to keep the bag off the bottom for applying heat and still have room to spare in the kettle. Heat to boil another 8-9 gallons in the HLT. Once boiling circulate through the lines, pump, chiller to sanitize. This volume gives me enough to sparge and a couple gallons for cleaning up after the boil is finished.
Pull the bag and let it drain, twist to speed draining, drop output hose into top of the bag and slowly pump water into the grain bed sparging until I get my pre-boil volume of 13.25 gals. When I start sparging the water is generally under 200 degrees, usually in the 180-190 range. No I'm not concerned about tannis from the hot water because I adjust the pH to 5.4 prior to boiling.
Am I concerned about tannis from twisting/squeezing the bag or as some have asserrted just allowing gravity to "squeeze" down on the grain bed as it hangs suspended? I haven't been...I've given it a good deal of thought but I personally have never noticed an issue. I'm not a 'super taster" by any stretch. No one else has mentioned it and I have a few buddies who are beer judges.
I have a ratchet lift to pull the bag from the kettle. Otherwise I couldn't do this size much less heavier bills for bigger bears. I use this exact process and about the same mash ratio for my 5.5 gal batches.
I get very high but more importantly predictable and consistent efficiency with this system and process. I'm happy with it and enjoy my brew days. It evolved from my 1 kettle BIAB. My 9 gallon kettle couldn't full volume mash all the beers I wanted to brew so I used typical cooler MT mash ratios of 50/50 first and second runnings. I'd boil water first and store in a cooler, then mash in the kettle and use 2L pyrex measuring cup to "pour over" sparge.