OK, here's an actual example of yesterday.
I had 2 old (in age, not use) 3300 mAh Turnigy Nanotech 30-50 C Lipos.
Just made a new model, aerobatic, 1.3 Kg. Wanted 350 Watts, and used a 3536-1250 KV with a 10*7 prop.
On testing with the old batteries, they gave 270 Watts @ 32 A,
and 9.1 VThen tested with a new battery. Got 560 Watts @44 A and 12.2 V
Massive difference
The old batteries are not puffed excessively, and all cells are in balance, and charge till 12.55-12.56 V
Just died from old age - however the important point is how the voltage can drop under load, especially on old batteries, and the importance of measurement (I did not measure thrust and RPM, did not need to in this case.)
And yes, the maiden was successful
Will be posting that log soon