Also, what will you be shooting? Slugs, Buckshot?
If you cut the choke off the barrel and you are using buck shot, you may just pepper the guy with a couple pellets. Might be ok with a 12ga, but I think with a 20ga and buckshot, you would want to keep the barrel choked to concentrate the pellets.
I think you should cut the stock down rather than the barrel. I am not sure what the legal limit for minimum overall length is for shotgun, but as long as it has a "butt" on the end and measures long enough, it would be legal. This would improve maneuverability without affecting the choke.