Doing it this way, also gives the APU hot parts a little longer to stabilise, before hitting it with the very big load of the pack(s).
I'd let the metallurgists take care of that. Or better yet, delay the APU start by those 3 minutes if you don't need the bleed yet and save the planet a little. 1 min = how much, 3 USD worth? People starting APU early to 'give more time to stabilize' fleetwide sounds like not overly bright SOHabit.
The EGT increase by introducing bleed demand is marked but whether or not it is real-life damaging becomes debatable. Keep the EXT PWR connected to mitigate that, why not.
'Over the lifetime' is academical truth because the supposed savings only arrive with the overhauls.
No dispute with the other reasons, your honour.
My simple rule on which hairs to split, when disussing small issues like this one:
Does this improve anyting for the given day/sector?
such as
- fuel burn
- noise
- workload
- complexity
- customer experience
- ontime performance