Board Thread:Fleet's Prepared-ness/@comment-30010835-20170704182712/@comment-26366865-20170705022017

From the Combat page:

Heavy/Aviation cruisers only require one fit gun to get their fit bonus, there are no overweight mechanics regarding cruisers. Keep in mind that the fit bonus only applies during a night battle and equipping more than one of the same fit gun will not give you any duplicate bonuses.

Thus, equipping a No.2 along with another cannon (preferably a No.3) will get the full bonus anyway. The reason I suggest upgrading the No.2 is that it's far easier to obtain the fodder to go past +6 compared to No.3. Though, as you said, upgrading Iwamoto and Iwai is indeed a good idea.