Temperatures below ISA will give you max power
What engine type are you talking about? I'm used to getting max rated thrust upto ISA+15.
From ISA to ISA+40º engine performances are gradually degraded because of EGT limits, that is to say "You can not use full power" then you use reduced or derated power always function of the EGT MAX.
Not really correct, if the OAT is 10/20/30/40C, that is still considered FULL POWER, its only reduced if there is a difference between OAT and the temperature used for the takeoff/thrust calculation.
Mutt