I believe we were long over due for the minimum change to greaters, so adding this change to the all in one would seem logical. Let word is greater here, normals and lessers should be lower naturally.
They could work like that, but they'd ruin that part of the market (manavamps/clumsies). Everyone would be using int potions to get less int gains, and those potions would cost like 100gp each only. If it wouldn't lower int gains that much, then there's no point, it'd be just another thing without any real relevance
If this was a thing i would use it, it would be nice for anyone not running with a mage sidekick.
Hell why stop there, make uber boost potions that boost a bit more than the standard forms but take balron scales or something to craft