Is it worth it?

If I have 100 sneak skill and then wear gear which says "improves sneak chance by x%" will I be noticably more stealth?

For what i've read,yes the skill pass of 100,only using Oghama Infinium...

I've heard of a bug where sneak past 100 goes back to low numbers, so like 140 sneak would effectively be 40 sneak. I don't know if "increases sneak by x%" affects this, or if its only through console codes that the bug happens.

No, it is not worth it.

I'm playing on 360 patch 1.2. After hitting 100 the infamous bug on the PC where Sneak reverts back to 1 at 100 did not trigger. However, I can tell you as an experienced assassin that Sneak works best around skill 50 onwards, with absolutely no fortify enchantments. Once your natural Sneak score is around 60+, very few things can detect you unless you stand directly in front of them when they are alerted. Save your enchantment slots for other stuff like fortify primary weapon (4* archery or 2h) or resist magic/fire/frost/shock. If you've maxed that out, put something like fortify health or just go with more resist gear.

*If you're using daggers, don't bother with fortify 1hand unless you're using swords. Daggers don't benefit from fortify 1hand enchantments, but the Armsman perks do help with base damage, and that is definitely useful when you're going for *30 multiplier backstab damage.

It's really not necessary.