I'll tell ya, love my bike but the stock tired are pretty weak. Looking to upgrade to 12x28, 8 in front, 10 inch rear, bit more clearance and a lot more umph! Any input or sizes you've tried would be awesome. Not getting wheels, I'm ok with stock rims.
2016 Sportsman 450ho

 

What tires are on it now? The PXTs?
I have those in a 26x11x12, and 26x8x12, and I really like them. Haven't had them through mud yet, though. Just used them in the snow. I know they rip my lawn up pretty good.
I know that if I went to a 28" tire, I would be doing some clutch upgrades.

 

I think the 450s only had 25" 489s but I could be wrong. I had those stock on my Trailblazer and thought they were good for general purpose riding on dirt, but they're not the best mud tire by a long way. Still better than some other stock tires I've seen. Besides a clutch kit I think you'll need a lift kit to get enough clearance with 28" tires. Possibly wheel spacers would help but I don't know anyone who uses them. You'd have to find out from someone else.

 

You are correct. 25s were stock.
I stumbled across a good deal on the 26s, so went with those.

 

I couldn't remember if yours was a 500 or 450. It's easy to put that in your signature if you want to. The 26" PXTs are what came on mine. My friend's Sportsman 1000 came with the same exact diameter and width PXTs but on 14" wheels. Our tracks are identical.