I remember the designer mentioning that even though the rules say you set all values as if you had one less investigator for each additional board after the first, this does not apply to the Great Old One. Does the 2 monster per gate threshold stay up as well, or can you play with 5 players, two boards, and only have one monster per gate?

I only play with one additional board at a time, but I do interpret the rules that if I were to use both at the same time, I would only get 2 monsters per gate if I played with 6 investigators. With 5 you get eggroll, but only one monster.

Quizoid said:

Within the game, you treat it as -1 player for all purposes. This is for teh two monsters per gate during the Mythos and encounters that count investigators and anything else that might pop up.

rthe only exceptions, as you pointed out, is the Final Battle. That reverts back to the actual number of investigators.

So yes, with 5 players and 2 expansion boards, only 1 monster per gate.