Does it seem to anyone else that 5 player is absurdly harder than 4 or 6?
Other than flat num-investigators effects 5 is just as bad as 6 which seems wrong. It seems like the rounding rule should be round down, min 1, for half investigator checks.
One extra player simply didn't cut it for double the gate spawns and 3-clue solves instead of 2-clue solves.
5 players is definitely harder than 6 because (as you point out) the same gates, clues and monsters are spawned for both, and mysteries that require half clues require the same number of clues.
5 players is harder than 4 because more gates, clues and monsters are spawned for 5 and , and mysteries that require half clues require more clues.
But, although 5 is much harder than 6, I'm not convinced 5 is much harder than 4 because (beyond a certain number of investigators) the synergy between the investigators kicks in.
Similarly,3 investigators is harder than 2 but its much harder than 4.
In short, the most noticeable lack of balance is between 3 and 4 investigators and between 5 and 6 investigators.
What, I've started to do to balance things out a bit is spawn one less gate (and its monster) on set-up if playing with 3 or less investigators and one more on set-up if playing with more than 5.