During AoE pulls with the dps outgearing you as a tank, it's common for a few mobs to pull off of you if they don't allow you the time you need to get agro. The purpose of the tank, though, is not to make sure all mobs are focusing on you, it's that you don't allow the mobs to hit the other members of the party/raid. If you are quick on the taunts to ping pong the mobs back to you before they hit the dps, you did your job. Granted, keeping them on you at all times is the goal of every tank, but while gearing up it's quite hard accomplish that goal with the rest of the group outgearing you and blowing up the mobs as they are used to doing with better geared tanks.
But on to the glyph question, I would suggest sticking with the Devastate glyph. I haven't read anything saying that it doesn't add the extra threat of the second sunder debuff, but I stick with it. I do know that Devastate adds extra threat over regular Sunder Armor since it has a damage component and, since I'm at work I can't verify this but, I believe the tooltip for Devastate states that it causes high threat.
Instead of switching glyphs, I would suggest working on your opening prioritization for AoE groups. How I do it might not be the same as others but it works well for me... Charge in, Thunderclap to gain initial snap agro as you're strafing a few steps to the side to get them in a nice tight group, Shockwave, and Thunderclap again when the cooldown is up.
Again, since I'm at work, I can't view your armory, but if you're not spec'd into Improved Revenge, spec into it. I macro my Shield Block with my Charge and Heroic Throw, so by the time I Thunderclap that first time, my Revenge is up which I use while I'm strafing to the side for my Shockwave. Once you Shockwave, tab through and use your glyphed Devastate once on each mob until, and after, Thunderclap is up again.
Tabbing through each mob to keep my sunders up on all as well as changing my Cleave/Revenge targets helps a lot, although sometimes the dps get frustrated with it because they can't focus my target, but I hold all of them to me so it works.
The problem with you not being able to see yourself amongst the groups of mobs or AoE spell graphics is something that you'll have to work on clearing up. Add-ons help with it (i.e. Miks Scrolling Combat Text moves all of the incoming and outgoing damage or heals texts to the sides of your characters, or where ever you decide to put them) but don't be afraid of moving the mobs as a tank. Just keep in mind that some mobs cleave, some have a conal AoE, some breath fire, etc... that you don't want to hit the group with. So stay in contact with them and move the group by backing around in a circle or something. If you see the back of your toon with no one behind you, you're doing fine.
Hope that helps.