Might as well answer before the lock...
The Gemini Game Elements converter works well, no lag that I've noticed.
But if you're looking for other Xbox native pads, the Mad Catz Beat Pad Pro is the cheapest ($25-$30) while also registering as a dance pad instead of a controller (Naki pads are cheaper, but show up as controllers in Ultramix).
Going the converter route, I've used a Cobalt Flux, an RO Ignition, BNS Ignitions, and the Konami PS2 soft pad with my Xbox.
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CF cbox mods and more at the 'Pad'