First of all, Spike Power is NOT the same as Firepower. Spike Power means: the maximum amount of Damage you can deal with one (1) broadside. Spike Power is important if you want to sink other ships quickly. Heavier cannons have more Spike Power, but take longer te reload. See Damage-Per-Second for more information.
For more information, look at the table below*.
This information is provided by the update that PotBS Wikia is undergoing, by Captain Vuur.