Do you need good sneak if you have 100 Sneak, are wearing all cloths, and have muffle shoes? Does it help? I currently add an extra 75% sneak to my outfit, but I can't tell if it really helps, or by how much. Does anyone know?

No, but sometimes you may encounter this...odd glitch...where the stats on your gear mean squat, and might get spotted left and right. un-equipping items then re-equipping seems to solve it.

to test it sneak around in daylight and see how much you get spotted. if your gear is uber, you'll practically be invisible.

if you want to check the buffs

tab, magic, active effects.

It seems to me like they add up beyond the 100, but I have no math to back it up.