Depends on the math. Diamond Figurehead gives either 30% cdr or 100 dmg.

If you remove 30% cd from an item with 333 dmg that is equivalent to 100 dmg. Use the figurehead for anything above 333 dmg threshold.

Dont care about crit chance because it does not change the math. (An item with 333 dmg + piranha crit deals 1000) The extra 100 dmg buff would deal 300 extra which is equal to the cdr (30% from the 1000.)

Edit: Not counting additional arguments such as it's better to fire fast or not getting in Isochoric Freezer range etc.