I actually played with incendiary gatling gun a bit yesterday. Was kind of fun. Especially after you pop their hull the first time. You can load up the incendiary and spray about their ship, causing happy little surprise fires to pop up. If you use it for the initial engagement, it actually might work better than regular gatling or the other ammo types. For just pure damage they'll just spanner and mallet the hull to keep the armor up. But with the incendiary they'll often have a knee jerk reaction to put the fire out. At that point you've already done damage and they haven't repaired it. During the cooldown on the fire ext you add more damage, possibly breaking the armor faster than anticipated.
It mostly depends on their crew and the crew's reaction time though.
So regular has 82 rounds with a damage potential of about 1086.5 damage in roughly 9.84 seconds
Incend will have 62 rounds and do about 821.5 damage in 10.63 seconds, on top of whatever damage the fire stacks cause
(Speaking of fire stacks, does anyone know how much damage is caused per stack of fire? )
You do lose some raw damage potential, but you do make them work a lot harder to make it up with the fire stacks. And if they mallet the hull, those fire stacks will just be sitting there eating away at it, even while you reload.
And as mentioned, chem spray will ruin your cunning surprise fire plan. But that's true with -any- fire based damage and weapons. At least with the gatling gun (and other weapons) they're often not expecting it as much. Especially if you bring a gunner who has other ammo types more often associated with those weapons.