Got a 09 500ho with 3k miles, the original belt was not slipping but with 3k miles I figured it wouldn't hurt guess I was wrong lol. The new belt has around 5 miles on it and is a Polaris belt. Only slips at wide open throttle. Still need break in time? I only opened it up one time and felt the slip...

 

Every time a belt is replaced, deflection needs to be checked as each belt could very a tiny bit in length. Look in your service manual for a belt deflection measurement and see if your is in spec.