I will answer from my perspective: I have no complaint with the performance of the stock tires. I've got 530 miles on my 2018 P500 now in all types of terrain, from Sand Dunes to low desert to high desert, sharp rocks, silt, hard pack, asphalt, mud, gravel, snow, you name it I've done it. The tires work well, FOR THEIR SIZE. It's my belief that they are your lightest option which saves horsepower, and perhaps even the softest option because they are so thin/compliant. But here are the reasons I got new "SunF Power II" 26-inch tires in the mail yesterday:
1. I think the stock tires look silly on the machine, because they look too small. I think the P500 looks TONS better with larger tires. However, looks are subject, so that's just my opinion.
2. When I rode Bulldog Canyon AZ with my dad several weeks ago, we rode a trail that is beautifully suited to the P500, but it drug bottom on rocks about every 30 seconds for 6 hours straight. It's like most of the time it ALMOST cleared, but not quite. I run my suspension at its softest setting, and my tires at 9-10psi, so with two adult men the clearance is affected. I'm hoping that with an additional inch of clearance I will miss MOST of those rocks next time we hit that trail.
3. I have noticed that my factory speedometer is a little off, telling me that I'm traveling faster than I really am. I hope that with 26" tires it will be more accurate, and when it says I'm going 35 mph, I'm actually traveling 35mph, not 32mph.
4. I don't have anything to back this up, but I believe many of these 6-ply aftermarket tires will be a tad more resilient in the harshest trail conditions. I spend a lot of time in very rural and secluded areas, so I want the added peace of mind of MORE durable tires. That being said, the stockers have not failed me.
Those are a few reasons.