User blog comment:Hayashi H/Advanced Equipment Guide/@comment-33019381-20160110220100/@comment-26486243-20160303131451

While these are like an endgame state, in practice it's usually better to stick with max★ 15.2cm first, then burst-upgrade it to like 6★ 15.2cm Kai at one shot when you want to upgrade, or else the intervening upgrade stages would actually be inferior to the maxed state of the previous gun.

Having at least two extremely high-modded 15.2cm primary variants is very important as CLs are compulsory for combined fleets - unless you have multiple Abukuma Kai Ni to get around ship locks, an artillery spotting CL variant will almost definitely be used. 4 would be better and 6 even more so, but you have to weigh this against upgrading other equipment pieces.