There is no way to change power settings on that particular VTX.
The power variance comes from component tolerances and antenna matching being an average to cover all the frequencies in the band. Other wise you would have to have a matched antenna for every frequency to optimize it. So the further away from the center frequency you get the less power gets radiated even though the same output power is used.
Someone did some testing a while ago on a 200 mw vtx and the output power varied from (Not sure if he measured in mw or dbm at the output) 35 to over 300 across the entire range of frequencies that particular model was capable of.
Here is a test to show what I mean on a 25mw vtx just to give you an idea.