imo, all goblins should be able to sap, the dwarven static defences would prove to be next to nothing unless perhaps if a military grade wall was built with stone blocks
This would make seiges extremely lethal, and relying more on military might than simply waiting them out. To balance this extremely game changing and unfair [sapper] tag, imho the goblins shouldn't know where you are in the first place, among the many cliches that follow around dwarves, one of them is the whole idea dwarves living in some hidden city that is of course built underground.
imo, goblins wouldn't even have a slight chance of finding you if your fortress is miles away from any gobo civ, however, if you try settling in the hills next to a gobo civ, you had better have stealth a top priority, the less you go above ground the less likely they will find you
In addition, damming or tapping into brooks or chopping down trees in a slash and clear fashion will prompt an investigation by a gobo squad. If they see you and make it home alive you now have a very angry gobo civ that will likey make war upon you. To prevent gobo armies sighting and overrunning little 7 dwarf forts I would think that more citizens increase the chance of getting spotted, caravans getting ambushed and betraying your fortress's location would be cool.
Tolkien readers, think Gondolian