I also have the NT aluminum trailer, which in my case came with 12″ wheels. I didn’t want to be one of the dead trailers at the side of the road with burned-out bearings, so I bought a set of what NT calls “high-speed” hubs, and got a pair of 15″ wheels/tires. I had already planned to get a pair of fenders that would accommodate the larger tires, but when I tried to mount the 15’s I found that the tires rubbed the frame of the trailer (no good deed goes unpunished). so I bought a pair of 1 1/4″ spacers which bolted to the hubs, and in turn mounted the wheels onto the spacers.
I asked NT what the speed rating of the High Speed Hub was. They replied that hubs were not rated, only tires were rated for speed. I asked back “Then why are they called High Speed Hubs.” They have not replied to that yet.