Right now it seems like only the number and size of barrels determins the stats of a loadout.
A two tripple turret setup of a certain gun will have the exact same stats as a three twin turret setup. The only diffference is the appearance.

My suggestion would be for maingun setups with more barrels per turret to have less weight (i.e. less speed-loss), but also lower reliability than a setup with the same number of guns spread to more turrets.

A turret with more barrels was usually more complicated and thus more likely to mulfunction than a similar one with fewer barrels. Also if just one turret gets disabled, it'll disable a bigger portion of the overall firepower of the ship (Disable one turret on the Dunkerque or Richelieu and they lose half their main battery. Disable one turret on a Bismarck, Admiral or Kongou class and they lose only a quarter of their main battery).
On the flipside having fewer turrets means less weight.
The speed buff and reliability debuff seems to me a good ingame representation for those real-world issues.