When I don't have store TP roll and I'm using 5/5 kenda on apex mobs 25% of my dmg is autos. Kannagi AM essentially is 50% auto-attack damage, thus I gain 12.5% damage from Emp AM3. Kikoku aftermath is essentially 10% damage if benefiting from the attack power.
If I take off kenda and use optimal pieces for DPS(Adhemar +1 gloves, samnuha tights) and get sam roll, my auto-attack damage plummets even further lowering the power of Kannagi's AM.
50 AGI is a pretty irrelevant stat and does almost nothing besides racc, while 60 attack will add more damage, so Kikoku wins here. Augmented, 70 agi is still pretty not useful outside of racc, but the 20 dex does indeed help. 60 attack and 20 dex can be considered equal.
Basic math shows that Kannagi is BARELY better than Kikoku ignoring having to WS at 3k and a fight lasting 3 minutes to take full advantage of the AM.
So from this, I'd just say "make whatever is easier/quicker until you get Heishi" because the difference between a RMEA katana and non-rmea is absolutely massive - Make Kikoku. Kannagi, in optimal situations, is barely better than Kikoku and requires massively more time, gil, and effort.
Is the TP bonus that broken with Katana?
Outside of Ten there isn’t a lot to bonus.
Shun - att bonus
Hi - crit rate
Ten - huge scaling
Kamu - acc down
Ku - nada
Metsu - AM duration
Jin - crit rate
Am i missing any big ones?
50% attack on shun is huge on an attack starved job.
Ninja lacks a good high damage spammable WS, 750 tp bonus(moonshade + heishi) makes NIN have a way to contribute to TP burn situations. Without Heishi, ninja's DPS is pretty meh in any TP burn situation as Ninja really relies on skillchaining to do any decent amount of damage with it's excellent SC properties(a good fusion ws, a good gravitation ws).
Edit: Augmented Kikoku also ends up giving NIN a VERY strong 8.4 ftp spammable WS. This makes Kannagi even worse off as a weapon, as 10% dmg to a weak ws(blade: Hi) cannot compare to another 20% on a very strong weapon skill(metsu).
At this point, I can see Kikoku beating Kannagi even if you are pdif capped and not benefiting from the AM or the 60 attack on the weapon. Augments have simply made Kannagi fall behind even further.