after 300mf, anything above that has diminishing returns meaning clear speed is more important so you can get more runs done. after that you need more players in your game vs solo, you will get more drops in even players 3-4, doesn't have to always be players 8.
I guess I have a hard time balancing survivability when I more than one person in the game with my MF. I am lvl 92 blizz sorc and can do Baal runs with full party no problem, but only because they come in to help at some point. I feel like if I am MFing on my own in a full party I will get wrecked.