Of course, this is exactly the difference between a derated takeoff and an assumed temperature reduced power takeoff, not the Vspeeds but the VMCG.
Boeing states quite clearly that a derated takeoff power is to be considered the maximum power for that takeoff, unlike the assumed temperature reduced power setting. But then again the assumed temperature method employs the maximum power figures for applicable VMCG calculations, whereas the derate setting uses the actual forecast derated power setting for VMCG calculations.
All this of course goes out the window if windshear is encountered, which sounds like what you are suggesting.
Cheers
EW73
