1515 1.5d or 1512 2d ?

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when you higher voltage amperage ratings become less of a issue

That is partially true, but it really depends on the driving you do and your gearing as a motor is capable of the same amp draw across any voltage. So stop and go can be just as torturous on 6s as it is on 4s. Your avg amp draw will be less over a full run expecially on high speed passes, but spikes will remain pretty much the same.