You're oversimplifying by assuming that native Dual Wield is the only factor. Koga is better than Thaumas/Iga largely because in slots with no-brainer haste options (Head, Body, Hands, Belt) you're already at 21%, and the only feet options worth considering give you another 2 to 4%. In any non-solo situation, the difference in attack speed between Koga and Thaumas/Iga would be overwhelmed by TP considerations and other stats without the gear Haste cap. It's possible this belt changes things.
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