upgraded to 14"s tires as well
As did Curtis, I also switched to 14" tires on my 2011 X17C and am also running Kumhos - 185R14C. They are radials and rated for 1875 lbs. per. Prior to that I had 2 sets of bias plys. We had put more than 20,000 on the two previous sets and as others have advised that's probably not bad for trailer tires. I did check them at every stop and had to watch the wear closely. I can't find my axle documentation but I believe it was rated for 3300 lbs. and we run as far below the gross of 3500 lbs as we can.
We just completed a trip of 2000 miles on the updated setup and I couldn't be more pleased. The tires came back still looking new and they towed well. Honestly, I'm not sure there was a huge difference in towing but it was hard to judge because we had the worst towing conditions I've yet encountered. Going we had 25 mph head winds gusting to 35. Coming back we had 17 mph tail winds until we hit Indiana when they switched to the west and intensified to 30 mph for a nice little side wind. I run a Reese dual cam WDH with sway and I have had a couple of friends laugh at my overkill for this small camper. They may be right but in spite of all the wind she followed straight as an arrow never wiggling once.
I would not go back to the original setup though I didn't like forking out the cash for 3 new rims. As Curtis said, there is still plenty of clearance. Without a doubt, the key to improved wear is weight regardless of the tire. We lighten the camper by moving as much as we can to the TV and other little things like emptying the hot water heater. I kept the 13" tires and plan to mount them back on when the camper will be stored for long periods to save the weather impact on the radials.