IMHO, that's a legitimate, very smart question.
As an engineer with some experience, though not with this subject,
my guess would be that it is a relatively simple exchange of energy. Knowing that I'm not an expert, try these ideas...
1) Take off power, at max, flex or any other standard is high. TO thrust comes first and packs second.
2) TO thrust + pack(s) requires more energy than TO thrust alone, thus placing a heftier demand on the engines.
3) Most take off procedures are performed at far less than Max Thrust, thus there is some wiggle room. That said, if it becomes necessary to use Max thrust - Firewall the levers (and pray), every ounce counts. Do you want to be flipping switches to shutdown packs and hope for the max possible motive energy?
It has been a while since I've flown on an energy-limited airplane, but some flyers still kill the packs, if briefly, during the TO roll. That may not be universal, but it remains common. One safely flying and starting to 'clean up' for the climb, the packs come back on. Even when shutdown for the TO roll and into a stable climb, the packs are off for 2-3-4 minutes. No one will suffocate, freeze or cook during those minutes.
I So NOT Know how much energy the AC Packs consume, as a percentage of energy produced by the engines. If only for engine health, shut the packs down during brief periods of high demand and return them to service when the energy needs are reduced. That's not flying procedure as dictated by the Chief Pilot's office, but simple energy conservation, common sense and of course, retaining the option of using 105% for thrust if necessary.
If I'm missing anything, please let me know. I am not criticizing, only suggesting that during those Critical Phases, the drivers should have 105% access to everything necessary. Darn good Question (even if I bent is a bit...)
