1. Sufficient stamina? Only when mine equals the LK
2. Any increase will be so trivial that would it matter? Really, isn't the answer "DPS, get off your lazy butt and earn some gold for crated gear..."?
3. Vig him. What's your rotation and compartive gear? Don't rely on a shield enchant.
Really - 1 is in jest. But points out the issue. You can never have too much. 18 stam = 248 HP (Kings/Sanc, ICC buff, warrior bonus). What's your damage reduction? Assuming 66%, that's roughly 750 extra damage you can take from physical sources before dying. Just 248 versus magic
18 stamina isn't trivial unless you've never been hit from an overkill <750ish.
You're basically trading 750 EHP against physical attacks for a roughly 20% (whats your block %, 20%ish?) chance to block between 81 (regular block) and ~350 (SB, SM, and crit block - forgetting anything?) incoming physical damage? Even if you were to block, the 18 stamina would give you 2x as much benefit (if I understand the math correctly) than the block enchant.
If you're concerned about threat - do you glyph devastate? That may be a better option if you're really concerned about threat.