It's a bit initimidating at first, but if you just want a quick reference of "this is better than this" you can just use the Gear page and the Setup page to input gear/buffs and it'll do all the work for you.
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when people say "this is the best" they mean in certain situations like capped haste two bards, a corsair and someone shooting acid bolts at the decent challenge mob you're killing
That is actually the precise reason Koga Chainmail +2 isn't very useful. It has essentially no situational uses.
I've mentioned in the guide that if you don't have a Thaumas Coat, it pretty much always comes down to either Ninja Chainmail or Iga Ningi +2 (I think Usukane Haramaki +1 might be a contender now if you're getting set bonus). Ninja Chainmail for capped accuracy and such, Iga Ningi+2 if not. It's true that people like saying "***is situational", but I can't think of enough situations where you're saying "This monster has just enough evasion where Koga Chainmail +2 is winning but I am also not benefiting from the STR/attack/critrate that Iga Ningi would be giving me" to actually bother building the thing and then lugging it around unless you know specifically ahead of time that you're going to fight a specific enemy where Koga +2 is coming out ahead.
If you're buffed out the wazoo, you wouldn't need much DW gear at all and you wouldn't use Koga Chainmail +2 or Ninja Chainmail anyway.
If you're wearing Iga Zukin +2, Suppa, Koga Legs +2 with 21% haste plus 15% haste spell, your attack speed is:
0.48 x 0.64 = ~0.307
Of what it usually is. 3% and 5% DW affect it thusly:
0.45 x 0.64 = 0.288
vs
0.43 x 0.64 = ~0.275
In layman's terms, you're attacking 364 times with Ninja Chainmail in the time you'd attack 347 times with Koga Chainmail +2, and attacking 326 times with something without DW/Haste (Iga Ningi +2 in this case).
Let's ignore attack for a second. 15 accuracy from Koga Chainmail +2 is 7.5% accuracy. So let's find a point where your accuracy is low enough so that 15 accuracy is going to matter more than 2% DW.
We want
364x < 326(x+0.075)
38x < 24.45
x < 0.6434
This is, your accuracy has to be 64.34% or less before the accuracy of Koga Chainmail +2 makes it win over Ninja Chainmail.
Now obviously with the attack factored in it changes quite a bit in favor of Koga (And Iga+2), but I hope this helps you understand just how powerful dual wield is for Ninja. It's conceivable for you to find situations where Koga Chainmail +2 might win out over Iga +2 because you just happen to only need the 3% DW and not the 5% DW and you are also capped fSTR and dDEX, and you are also needing exactly 15 acc and 20 attack and thus the 3% crit is the only edge Iga Ningi +2 has. These situations do not come up often (if ever). This is why I wouldn't bother bringing my Koga Chainmail +2 around with me anywhere. It looks really swell in town, though.
To clarify, Koga Chainmail +2 isn't a terrible piece or anything, it's just kinda pointless to go to all that effort to make a sidegrade piece like that when Ninja Chainmail and Iga Ningi +2 will serve your purpose at both ends of the spectrum as needed.