I had a chance to do another trial ride with the Epics. I decided to keep using the two different boots. One with metal teeth and the other with the comfort strap (no metal teeth). This time I added some duct tape to try to protect the hoof wall from the metal teeth on the one hoof.
I also stepped up the speed a lot. I did 8 miles in 52 minutes (instead of 2.5 hours) and also had a warm up and cool down.
The results were still encouraging, except for Tanna changing his gaits. He was reluctant to speed up into his working trot. I'm hoping that just some more rides will help him get used to the boots.