As JustAnItch said the legends are not 50/50 at all if you count their cards, but even if you don't, they are not 50/50, they have a 60% chance of going +1/+2 through a corner. There are also plenty of cards (Brakes, Gas Pedal, etc) that will let you guarantee a slipstream from them.
An extra player would mean longer queue times, longer races (more relevant in real-time games, but also would make turn-based drag on), less wins for everyone, and more elo swing if you get lucky/unlucky games.
Ultimately though, 5 player games does usually get suggested each season in the gurus, so it will just depend on if people vote for it (plus if you just play lots of games Sombra, you'll get into gurus and get to vote on it yourself)