Derate and assumed temperature are not the same thing of course.
Derate is like having smaller engines, whereas assumed temperature is like having the original sized engines but not at full throttle. Therefore, depending on the figures; your assumed temperature example might result in more power than your derate example.
You quote 24k for each case, but the derate case should use a lower power, so I am slightly confused by your starting figures - possibly my glass of wine is clouding the issue!