The drop in heavy guns was after Admiral D Artagnan suggested that a kk with D6+6 firepower would do way too much damage at close range. This proved true in playtest (which I'm still busy publishing) So the heavy guns were dropped to 4, basically it's a trade off for longer range wbs. This resembles larger vessels, who all have identical heavy gun strength in every arc. Also it justifies the torp upgrade more.
The costs are determined by a modified smotherman, where the cost of heavy guns is only 1.5 rather than 3. This is true as 1.5 points is the cost of 1wb at 30cm. Presumably 1 wb at 15cm would cost less (about half). Heavy guns are not equal to 2 wbs, and are actually worth about 1.85 guns. Therefore the assumption is that 1.5 is a reasonable price for heavy guns, and makes a backbone for the KK to be built on.
The str 4 heavy guns mod was maintained for all options for consistencies sake. To keep the factors to a minimum.
The KK is overcosted by about 12 points according to this formula, therefore a drop by only 10 points is warranted, as well as an increase in weapons strength by 3 points (+1 prow wb), if the heavy guns are dropped to 4, then the side guns can be increased slightly as well. The stats could stay the same and go to 15 potentially, but the community wants increased firepower more than they want reductions.
There is also a desire to have more consistent weaponry, and this is done through adding non-variable stat. So the larger the non-variable stat is, the more reliable the ship is.