Just checking, instep means arch right?
I have high arches myself and almost had to give up the sport. I quit ice hockey and roller blading as a kid because of the pain any tight boot would cause me, but I would try every option before quitting snowboarding.
I once got expensive insoles from a foot doc but they didn't do squat, although any heel insert that raises the outside rear part of your foot so that more weight is placed behind your big toe should help. I also got a bunch of custom foam inserts on my old Salomon boots, from L-bars to shin foam inserts, but I still had to take my boots off every hour because it felt like knives were being stuck into the sides of my feet.
What finally got rid of my problem was sizing up to a 10 from 9.5 and getting a Salomon boot that had that strap outside of the inner boot that would keep your heel from lifting (it crosses your foot and sticks to the top of the boot). Basically, there is one boot out there for you that will help so just keep trying them on until you find the best option, you can always get some custom foam inserts as per recommendations from your local ski shop guy.