I achieved 87% efficiency on my last BIAB brew using re-circulation and stirring during the mash with a 3:1 grain to water ratio. That's 10% better than what I achieved with batch sparging. You'll have to try your system and see what it gives you, then use that percent efficiency for your next brew calculations.
Tannin extraction occurs when pH increases above the ideal mash range (5.2-5.6). This is often a problem during batch sparging as the wort is diluted by circumneutral pH sparge water, causing the pH to increase unless the sparge water is pre-acidified.
Assuming that your BIAB mash pH is in the ideal range at the beginning of your mash (if not, add acid), then it should remain stable regardless of whether you squeeze your grains, and tannin extraction is a non-issue. Mechanically disrupting the grains (i.e. squeezing) is not going to extract any more water soluble molecules (i.e. tannins or sugars) than would one hour of soaking in ~150 degree water. I love how BIAB greatly simplifies mash chemistry!