Unless you make firearms "uber deadly" you can't really make them more useful. The advantages of modern firearms is that they can kill people before they can get close. In a D&D/PF like setting this isn't the case as because of the HP bloat everyone can easily take several shots and get into melee range anyway, especially as even in modern games combat tends to happen at very close quarters.
So either firearms need to be a lot stronger or they must make it harder for enemies to close into melee range by disrupting movement.
And imo, making sidearms AoO free will likely result in everyone using sidearms as this is the primary weakness of firearms. As RangerWickett said, with that rules you just have created melee firearms because the +4 to hit is just too good to pass up as compared to an actual melee weapon.
