You should gain slightly more LP if you get an S+

Knowing that you can stomp the ELO you're at with constant S+'s yet only gaining 18-22 LP per game is pretty depressing, especially since losing a game will put you back down where you were until you win 2 more games.

An S+ rank should give 3-5 more LP, since it's extremely hard to get and means you stomped insanely hard.