I honestly don't know if they do need any kind of boost. I think that the fact that they let you consistently deal level-scaling damage that adds weapon specialization is awesome. However, they are a significantly worse option for dealing with a single tough target. While it is fine for this weapon to have this niche, I feel like a lot of people who would be drawn to "shotguns" would be people that would want to deal a lot of damage at short range, and I was wondering if there was a tweak that lets them fill this role with the weapon they want.
What if you gave shotguns the ability to toggle to a narrower spread as a swift action, changing its damage to about the same as a like leveled rifle, but it still has the same range increment of the cone attack (it can fire beyond this range, but in most cases penalties for range would stack fast). Any sort of cover provides full cover against weapons fired in a narrow spread (it's impossible to fire around people).
So in this case, the possible damage output for shotgun weapons would be the same as assault rifles, and they have the alternate option to deal lesser damage in a cone, at the cost of "you will often take a -2 to attack from range penalties, or else have to move and give up your full attack."
Well, for the range increments, I wanted avert the trope we always see in video game where shotguns are useless past 15 feet. I mean people use them to shoot clay ducks, so I figure if my PCs want to take a -6 or whatever penalty to fire at something that fire away and still use damage that's no higher than other longarms, it should be okay.