If delayed as an armament sailor until lvl 120, they would need full experts and 180 vets to reach the reload cap, and would have a little over 1,000,000 repair TA.

Through all this time spent delayed, you would not have access to any of IJN most powerful AA guns, like the 2x 4.7" A, you would have to make do with something like the lvl 22 2x3.9" D which have very short range.

Check this out.

http://navyfield.com/Community/Forum/View.aspx?num=18497&searchtype=0&pagecount=0&searchvalue=&sort=5&category=A02&page1=1
 

Although the link above only talks about UK gunners, I believe you can reach Acc and Rld cap with Elite Repair for IJN as well (although IJN may end up with less Repair True Ability due to no Repair boost as national trait). 

However, the point of late-classing is to keep as much Repair ability as possible, so it does not really make sense if you want your gunners to reach Acc/Rld cap at Lv 100. In other words, if your gunner reaches Acc/Rld at Lv 120, then your gunner will end up with more Repair ability.

If I were you, I would use a pair of Elite Repair sailor to:
Level them up all the way to level 100;
Class them to Chief Gunners at Level 100 all at once.

Then at Lv 120, your gunners will have (after applying Premium Sailor boost item):
Around 1757 Acc/Rld ability;
Around 1676 Repair ability, and
750 recuit total

If you give them 40% vet, which is 300, then you'll have:
2.9 M Acc/Rld True Ability, cap reached;
2.76 M Repair True Ability for one gunner; 5.5 M for the pair.