I've followed this whole wheel upgrade debate for some time now without commenting. The forums are great for learning about anecdotal data based on real world experiences, but I tend to believe the squeaky wheel gets the grease and complainers tend to dominate the boards. That is to say things aren't all as bad as they are made out to be. Case in point GYMs and 14 inch wheels.
Have been perfectly satisfied with both, but would not consider upgrading wheels...Do they make the trailer roll any better? Thats debatable. Marginally higher, yes, better? And as you have pointed out the wheel wells are about the same and if the blowout rate is roughly the same for tires of different size (independent of the whole brand and type, ST or LT,debate), wouldn't larger tires cause more damage to the well and the surrounding trim and skin?
I believe proper tire maintenance is the key and as such, as I am planning a major cross country trek, I just replaced my GYMs (I'd rather not get into the debate on what brand). I will say the new tires are not GYMs but I will also say the GYMs have been trouble free for me.
For me, I chose to have that work done by Discount (Americas tire), and they did a great job. I like the free rotation and balancing and yes I did buy the road hazard coverage so blowouts are less of a concern. I wanted a tire that was readily available and an installer that has a great distribution network. Perfectly happy to stick with my 14's. Next job up is having the bearings repacked and and brake inspection and we'll be ready to roll. New tires all around on the TV as well.
I guess if I had an issue with an axle, I'd consider upgrading to the neverlubes and may as well go with 15 inch rims at that point, but I'll cross that bridge when I get to it. Not one to spend lots of extra for marginal gains.