OK more data.

First:
Flight time on stock 800 mAh battery pack, well-aged (47 recharge cycles since new) with DD tail motor system: 10 minutes (8:48-8:58) of hovering and orientation practice in normal mode. So that's about 2 minutes less than I remember with the stock system, but I'll swap out and check again. Keep watching.

Second:
No lift-off on crash repair, requiring pitch adjustment: I've seen several people reporting this problem and I believe I have found a possible cause. I have carefully measured several main shafts and I found that the height from the top of the main gear shaft boss to the tiny hole for the head-retaining shear pin can vary, in my small sample, by as much as 0.040 inch (~1 mm). That may not sound like much but it is this distance that determines the collective pitch adjustment of the head, and it can translate to enough pitch change at the blades to be noticeable in performance--like preventing lift-off or requiring a lot more or less throttle to lift off.
The way to fix it, of course, now that I understand it, is simply to lengthen (or shorten) all three servo link rods evenly by one full turn. That should adjust the collective pitch evenly to compensate for differences in the height of the shaft hole.
Of course you should check all this with a pitch gauge if you have one.