For everybody that uses the gat mortar combo I have a secret to tell you. Burst ammo does more damage than greased in the mortar!
True, greased ammo reduces damage by 20% and also reduces projectile speed by 20%. On the other hand, it takes almost twice as long to empty a clip with burst rounds (8.4 seconds for Burst vs 4.6 seconds for greased)
Burst ammo damage per clip is 1764
Greased ammo damage per clip is 1411
If we include reload time and time needed to empty the clip, we get the following DPS:
Burst: 210
Greased: 316
Of course, the reload time shouldn't play a role as the enemy ship is probably going to be destroyed after one volley
Hull health of all ships for reference:
Galleon: 1400
Goldfish: 1100
Junker: 400
Mobula: 700
Pyramidion: 700
Spire: 750
Squid: 850
What plays a role however is the time it takes to rebuild the armor again, and that's where the burst rounds have a disadvantage.
These are the spanner hits it takes to rebuild the armor on the different ships according to Mattilald Anguisad's rebuilding reference:
Pyra Armor: 9 Buffed Armor: 10
Galleon Armor: 10 Buffed Armor: 12
Junker Armor: 9 Buffed Armor: 11
Squid Armor: 4 Buffed Armor: 5
Goldfish Armor: 6 Buffed Armor: 7
Mobula Armor: 8 Buffed Armor: 10
Spire Armor: 6 Buffed Armor: 7
I think one spanner hit takes one second (not sure though), that means that theoretically the armor of the Squid, Goldfish, Mobula and Spire could be rebuilt before a burst mortar clip could be fully emptied.
Still, I am not entirely sure whether greased or burst are in fact better. Greased has an upper hand in fast, close-range engagements I'd say, but it strongly reduces range and and accuracy due to the slower projectile speed.