I have been buying from EPI for years. Good company.
But I have always found that OEM belts are best overall over aftermarket.
If your getting some slipping, there is something causing it that may be or not related to the belt directly.
Over equiped machines or heavy riders can cause a properly runing machine to get slip , more so during a high traction situation. Any slip at that point does damage to the belt and it just gets worse from there.
Dirty and / or glazed over sheaves can be a common culprit.
The secondary's helix can get stuck so it dosent shift out properly and loose tension on the belt.
Secondary spring can loose its tension.
Any oil or grease contaminating the belt will cause slip.
Improper torque on the primary will cause misalignment of the clutches can lead to sliping.
Deflection may be off, thats a common sliping cause.
Something overlooked with new belts is the minor coating of mold release compound that is present on every belt made irreguardless of brand. I dont know of any shop that will wash a belt prior to installing them. Left on, that release residue will cause glazing to appear quicker than normal. Most shops also clean the clutches with compressed air, which is the worse way to do this. Compressed air just pushes the dirt in further to the working parts of the clutch. They know this, and for most owners its a return trip to the dealer for service work.
The cvt clutches can be kinda tricky.