Another in the fight against discrimination!
Haha sike, yeah I got flatfeet too. I've been wearing arch support(orthotics) for a few years now and before I start ranting, I'll tell you that I take the sole of the liner(in my motos, its black/red) out and put in a much stiffer arch support.
Just a quick note, during football, I discovered that I needed to wear them in every shoe(practice cleats/game cleats/street shoes). From my basic understanding, the arch in your foot enables you to have a more elastic spring in your jump and to absorb shock, basically like a spring. If you think of sprints, from a crouched position, I was one of the last getting off the line without supports. For snowboarding, you(or I) could make the connection that your jumps and impact will be affected.
The inner-leg shin splints, I have zero idea why your getting that from snowboarding, I only had shin splints a few times in my life, during football in the middle of two-a-days. I had pain on the bottom of my foot, it felt like the tendon that runs the length on the inside of my foot would be stretched(very fucking painfully) apart whenever I had my foot flat, in the middle of walking/running/etc.
The difference in cushioning that the factory support in the boot(inside the liner if you have a line) will be noticable if you don't wear enough socks, but I'm sure your used to that after 12 years. The size 11 support felt like it was a bit big inside the liner and was pushing the edges out, but you'll break it in after a few runs.
If any professional/doctor/admin tells me I'm wrong and gives evidence, go right ahead. I'm just stating my experience.
Yeah, sry about rant