Here are my totally unscientific opinions:
top speed? No noticeable difference (hey, it's still a TW).
power to climb grade? No noticeable difference (again, it's still a TW).
MPG? Probably slightly less (but I never check my mileage).
What tire is that? That tread pattern should be smoother and quieter than the more aggressive ATV tires like the Duro.