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You're saying that 48v, instead of the 5.5v, should be used to calculate W ?
Then it would be 288W.
But still a null point, considering 1) max amperage of driver, 2) no room for a 1:3 pulley.
3) last bit of money I'm throwing into this setup.

I had a closer look at the specs in the link you gave, it says 6A / phase and 2ph, so it would be 12A. And it says 4.8V rated.. That would be 57.6W.
I'm just guessing here, but I seriously doubt that 4.8V is right.

If it works well, you're fine, but if not I would try more voltage by slowly increasing until performance is good enough.

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