Even for gen3+, the changing of Hidden power only mutates the IV bits that determine the type; they don't change the power value.
For both gen3+ and gen2, you can get a max power arrangement by first maxing the IVs (shortcut is to control click the Randomize IVs button) then change the IVs after.
Changing the behavior from only-mutate-type to mutate-type-and-max-power would remove the ability to change just the type. Plus, there's implications for gender in gen2.
If you want max power, just max the IVs and change the type after.