First gather your surplus gamers, box them neatly up, poke some air holes and ship them to me cuz I can use them.
I'm at a loss really. Havent had to deal with more than a total of 3 in many many years. Are you all actually playing the same game? What I mean is could you break into two groups. If nothing else you could get another copy of MoM and have 2 games running at the same time. Now that would be fun!
Most of the time, on here, you'll find that people are trying to help the investigators (myself included). As you know, it is not just about threat/players ratio. The more investigators there are the more they can do for the simple fact that they cover more area and can team up. The game is not exactly about balance though it does try to maintain some. This game is much more about the story and most assuredly getting there is the fun.
Having more than 4 investigators is a major imbalance (imo). Not only for the reasons stated but it also feel out of theme (save for specific circs). Its less of an investigation and more of a free for all scavenger hunt. The dark horrors lose a lot of power when a mob mentality forms. I played 4-way chess for a while. It was fun enough but it still wasnt chess.
My best suggestion is not a long term one. You could write some scenarios that are themed toward having more people. Something like a dinner party with a seance gone wrong. Stories like Ten little Indians or The Masque of the Red Death would be a good place to start. Actually a mob from town could work as a story too.
Good luck, be sure to let us know how things go.