I haven't bought the DW earrings personally, aside from the fact that I haven't spammed Delve as much as others (or at all really), last I checked it was a small enough difference that I didn't really bother wasting another 2 slots in my inventory on it (NIN's pretty inventory-floody for me). You're right it'd be better though, and it gets better as you add more DW. If someone wanted cheap high il weapons for NIN it'd be Pamun obviously, though at this point I'm guessing Kikoku would be the best mainhand or lolNagi if you're AM3 (or you could dual wield REMs, because I'm sure there's probably at least guy out there that built 2 REMs for a gimp job like I did!). None lv99 or lower katanas are worth it anymore, pretty sure not even Kamome inside Abyssea outdoes the sheer DMG/skill+ increase you'd get from an ilv katana. Technically you could get that extra 5% DW right now by using those earrings plus that Uguisu DW katana, but the latter's pretty crummy. Kinda sucks they outdated that weapon right after it came out because it was a pretty cool model.
There's a lot of new gear I haven't checked so it'd take a bit of playing around to find the best March'd TP set for NIN, but who the heck plays NIN in a party seriously. As for Iga Zukin +2, it's sadly still the best-in-slot TP piece normally because of the attack speed it gives unless you're fighting something really, really evasive. In those cases you'd probably go with something with a lot of haste and statvomit though, like Whirlpool. Thurandaut+1 isn't really any good because it's DW+5 but only Haste +1, so you might as well just wear something with like haste +7 or something.
As for why it's 7% DW instead of 10% DW, the simple way of putting it is because NIN already has dual wield, so it's not just "10% off my TP return must mean I added 10% to the stat", attack speed in this game gives "increasing" returns so you would need less of the actual stat once you already have some to be swinging 10% faster. NIN has 35% DW naturally so that means if I get another 7% DW on top of that I'm swinging at 100-42=58% speed of normal. 58/65 = 89%, I'm swinging about 10% faster even though I only added 7% DW.
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