120 vac vs 240 vac
i have never seen any scientific data supporting that a dual voltage motor, configured for the higher of the two supply voltages, will operate with any improved performance.
IF a test were done that would have a dual voltage motor on the lower supply voltage and again on the higher voltage, with all of the correct circuit components to have the exact same voltage drop in both configurations, it would be extremely likely that the motor would perform the exact same in both configs.
the most common issue is, that in the lower voltage/higher amperage config, there is more voltage drop leaving less for the motor. if those situations were improved with larger conductors (less resistance = less voltage drop), the performance would improve greatly. motors suffer greatly when their power supply is not within 5% of nameplate ratings.
for all those that have expressed this concept before me, i am in your camp.
A few just can't grasp it.
And just because your device is directly connected to the breaker, only 6 inches away, with #8 still doesn't mean anything. A loose or bad connection, even at that distance can cause enough of an issue to affect performance. And rewiring for a higher voltage could have fixed a connection issue, not the magic of 240V.
I had a loose ground on the boat last weekend, would have sworn the starter was out from the sound of it, fortunately I figured it out before I replaced the starter. But I probably should have added another battery, shove 24V at that starter to use the extra set of windings...