I had never heard of that, but really why do you need a lower cooldown? Tweaking the burst count and delay lines can be done to give the impression that the ship is firing continuously, ever if the damage is only being applied every 4 seconds.

I made an Ability called.. Alpha strike..

Increases Weapon cooldown -1000% for 3 seconds..

this... Shoots ALL the caps guns in pretty much a constant stream, a 30 base dps ship does about 2000-3000 unmitigated damage.

so % cooldown from abilitys seems to have no Minimum cap.

I also have a few ships with weapons shooting every 2.5 seconds and I have not seen an issue so far.

Im almost certain there is no Minimum cool down as far as effects go, its possible this was removed in Diplomacy or Rebellion.

 its very easy to test, just make a beam weapon that lasts for 1 second with a 1 second cooldown, Almost sure you will get a constant beam, and the damage will be correct

If you just want effects (since damage is calculated over time anyways) you can make a weapon with a 10 second cooldown, with a burst count of 20 and a Burst delay of 0.5 seconds.    this shoots every 0.5 seconds.... ?