The problem with incendiary are not only its low damage and low rof, but also that guns are just too good. With normal rounds a gatling can still destroy almost any armor, a carronade still almost any balloon, a mercrury and artemis still any gun and engine.
Incendiary, are comparable to the Pyramidion side guns, rarely used and the damage output of the other guns/ammo are better, but have both and you will win regardless.
I have incendiary on Goldfish side carronades, with its heavy gun on the front the disable is rather quick and almost unbearable, add fire stacks to balloon, armor and some random components and the damage is unbearable. One could say a flamer achieves the same result, but if chem spray is actually up, the carronade will still do a decent amount of damage.
I only really use incendiary on carronades actually, for everything else I prefer heatsink, because every other case is probably about arming time and about what ammo the gunner is going to take. Gunners can't use chem, so heatsink over incendiary, usually.
Dont take me 100% but im rather sure that it is 20% for greased and something between 20%-30% for heatsink.
Heatsink is exactly the same as greased in arming time reduction, but with less damage reduction than greased.