Let me try and answer that.
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How much does my Shadowmeld actually help me?
I'll just go back to basics here for a second; You can imagine 'stealth' as a skill similar to lockpicking, mining, or whatsoever. Every level you'll automatically increase 5 points in it, meaning 5 points of 'stealth skill' = your stealth will be equal to that of a player 1 level higher.
Back to shadowmeld. To be a bit more specific; Shadowmeld itself no longer adds any stealth 'skill' (I'll name it like that from here on). A racial named
Elusiveness is the (passive) buff that actually adds stealth skill. As you can read on Wowhead, it's 'value' is 5, meaning 5 points of stealth skill meaning Shadowmelding makes you harder to see as if you were one actual level higher than your current.
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How much can I stack these bonuses?
The stealth 'cap' so to speak lies at 1 'yard'. Read trough Nabterayl's post over
here for some more details on that.
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Are there any other stealth enhancers?
Yes, a couple. For rogues,
Master of Deception is the most well known enhancer. Back when it was 5 talent points, it used to enhance 3 stealth skill per point, adding up to 15 skill aka 3 levels. As it is now, it simply adds 5 stealth skill per talent point, still adding up to 15 skill aka 3 levels. There's also a level 70 jewelcrafter's
panther which also adds 5 stealth skill. Aside from that, there's a level 80 enchant named
Shadow Armor which adds 5 stealth skill as well (if I recall correctly).