No, your ampacity derate can be based on the insulation temperature of the conductor, but the starting ampacity (before derating) has to be based on the lowest temperature rating of any connected termination, conductor or device...See 110.14(C).
So for example, if you had a 100HP, 460V motor (124 FLA), you would need a conductor with an ampacity of 124*1.25 = 155A. #1/0 90deg has an ampacity of 170, but 110.14(C) prevents you from using the 90 deg ampacity, so you would need a minimum #2/0 (175A @ 75deg.)
But if the circuit needed to be derated for a 100degF ambient, you would see that #2/0 THWN would have a corrected ampacity of 154. This is too small for the required circuit ampacity. However, using a #2/0 THWN-2 would give you a corrected ampacity of 177A. Using the 90 deg conductor for the derating would prevent you from having to increase your conductor size.