The difference between greased and heatsink is 5%. That about 10m for the lumberjack. Nothing that will matter. Specially not on a goldfish which can choose the engagement range much easier as wund said.
Greased rounds and heatsink clip are both -20% projectile speed; heatsink does more overall damage per clip, however the dps for greased will pop a balloon before heatsink. As the lj is a balloon popper not an explosive finisher for example, I tend to prefer greased as it will take out the balloon faster. Incendiary rounds are -25% here I follow the same logic as yourself that distance is negligible and not worth the -30% rate of fire debuff or the -25% reduction in clip-size. If they are close enough for that -5% to matter, a gunner would probably be better off bringing lochnagar. I also like to assume that fire is a non-issue for the enemy because of chem-spray.
The lumberjack does 50 direct shatter damage but 300 burst flechette damage, knowing that the actual balloon popping comes from the AOE, burst rounds are particularly useful against ships with armour covering the balloon such as the pyramidion. For me, burst rounds +20% clip-size outweighs the -15% rate of fire primarily because of its use as a control weapon, having an increased clip-size allows the gunner to target multiple ships and keep their balloons down.
My standard loadout for a lumberjack is lesmok rounds, greased rounds, and burst rounds. However, situation dependent I am inclined to switch out either burst rounds or lesmok rounds for lochnagar.
