From: DylanX570 | Posted: 1/26/2012 12:13:44 AM Muffled Movement gives you 50% muffle, and I believe enchanted Muffle Boots give an additional 50%.
Yup. Once you get 100% muffle, going higher makes no difference.
100% noise reduction: Silence perk Muffle spell Ebony mail (only while in sneak mode) Light armor boots named "Predator's Grace" which you can get inside Hag's End (never seen these myself but the wiki says they are there)
50% noise reduction: Muffled movement perk Any self enchanted muffle boot Shrouded boots/shoes (including ancient type) Cicero's Boots Jester's Boots 2nd tier of Nightingale boots
25% noise reduction: 1st tier Nightingale boots
And then of course there are the 3rd tier Nightingale boots (level 32+) that have a bugged enchantment that gives +35 points of muffle which translates into 3500% noise reduction.
Lots of discussion as to what has what level of muffle items have and tests run on it's effects here: http://uesp.net/wiki/Skyrim_talk:Muffle_%28effect%29