Ok thanks for all the thoughts guys... Some in retesting ideas. Indeed, it must predominantly be for performance/economy.
Just to be clear then, as long as we have the APU bleed on for the turnaround (ie aircon system on) then there should be no problem for avionics cooling turning the extract to override (to save our hearing), as long as it doesn't breach limits in the MEL (max 1h up to 38 degrees which, let's face it, is going to cover you for Europe).