MMR/LP and Duo Queue

I'm aware that if you duo queue that you and your partner are placed against a team that is up to a division above your "real" one.

My question is very simple: If you win, do you also get full MMR/LP gains for this win as if you defeated opponents a division above yourself? Or does the game only give you MMR as if you just defeated a "normal" opponent at your own MMR anyway?

I don't see the point of duo queuing if you don't get massive MMR gains when you win, considering the opponents it places you against. If I wanted to just get "normal" MMR gains, why wouldn't I just solo queue in that case?