I've used that galleon crew setup before and didn't care for it. The guns were generally destroyed anyway, so you then just have an awkward heatsink ammo on a crew member who could be using something else.
My issue with heatsink is that its too niche. If you preload it, you still have to use it, which generally means your first clip is underwhelming. Then a reload, and anything over 3 stacks is just a start to either letting the gun burn out and rebuilding, or an engie coming to get it anyway as the gunner whacks it every reload cycle.
Again, I don't particularly care about its other merits for reducing arm times or giving extra ammo. Those stats were put on it eons ago to make it less effective than other ammo at the time, save for its fire retardant. Now its (I believe) more often used for those stats, and not the fire retardant, because an engie can do the same job more effectively anyway. My biggest complaint about it is lack of protection while reloading.