As far as I heard from PTS people 15% is clearly noticable. I believe that the stadard sneak radius is 10m so if you have -3m it will be 7m which is pretty close to a player. If it stacks - no idea. Tamriel foundry has done some extensive testing on game mechanics and some things do stack and some do not.
Imagine you are doing 300 dmg with a crit then +15% would be 345 dmg
