Jumping in at the deep end

[Fyron]

The heavy mount is just a nice extra - unless you're going up against large heavily-crystalline-armored ships, in which case it's essential.

I would have to disagree. The following are calculations of Damage Percent / Tonnage Percent to show the increase in the damage ratio compared to the unmounted version of the weapon.

Large mount: 200 / 150 = 1.33x base damage
Heavy mount: 300 / 200 = 1.5x base damage
Massive mount: 500 / 300 = 1.67x base damage

Now to look at increases in damage between mounts:

Large mounted weapons do 33% more damage than unmounted weapons.
Heavy mounted weapons do 12.7% more damage than large mounted weapons.
Massive mounted weapons do 11.3% more damage than heavy mounted weapons.

While the increase from large to heavy mount is smaller than the increase from unmounted to large, it is still 12.7% more damage than you would do from a battlecruiser. This is quite significant, and should not be underscored. In my assessment, heavy mounts are most certainly not just a "nice extra," but are as necessary to use ASAP as large mounts are.