Impose is specifically deficient for megacorps for 2 reasons one major and one minor, both stemming from how megas interact with each other. the major is that you can not integrate another mega, so impose puts the full productivity of those pops out of your reach until the next war at minimum. The second is branch office competition.
Poltically impose still does everything else for a mega that it does for a normal, make a weakened ethically aligned ally out of a former enemy.
Imo it's a stylistic choice that megas work like this for war, your supposed to be a territoriality small, economically/politically powerful. empire with a bunch of semi independent bootlicker subsidies. Shop smart shop S mart. Never mind that you franchised out a bunch along the way, everything falls under your pyramid.
Where it falls apart in how the game play works out atm, wide is superior, AI allies are the paste eating kid from the third grade on a good day, and anything you can not directly oversee is wasted resources.
Good flavorful gameplay design, that sadly was a little off kilter at launch and only got worse since in terms of style vs reality. Atm the best trade empire is not a mega but a merchant guild with federation and mercantile.