I have 95 sneak on my thief but I still get caught trying to sneak up on bosses, do these last 5 levels make such a big difference (talking about master lvl).
Ok, I guess you pointed out my weakness right there, nearly no superb enchantments on my armor apart from using the standard thiefguild/Dbrotherhood armor with VoLsung mask while dungeon crawling.
The difficulty has nothing to do with Sneak. At 95 you should be able to run up their pant leg. The only perks I put in Sneak is just enough to get up the right side. With just 1/5 in Sneak, all you need is a 75 Skill and a Muffle spell.
If you have a skill of 95, it's your sound, that is getting you spotted. A Muffle of 1.0 is maxed. The Spell is 1.0, boots you enchant are 1.0, boots you find are 0.5. That renders silent perks useless.
EDIT : I only play Dead Is Dead. I play my Necromancer/Conjurers and my Assassins on Master, as they very seldom get hit.