Two Parter

I was thinking of switching out http://www.wowhead.com/?item=32473 for http://www.wowhead.com/?item=29068 (assuming it drops for me again, got it once in a previous/holy lifetime) and http://www.wowhead.com/?item=28747 for the badge http://www.wowhead.com/?item=33523. Based on my armory (and assuming I might need the stamina trinkets (waiting to turn in my Furies Deck) would the extra spell damage be beneficial? It looks like I can make the change without hurting my uncrit/uncrush. Here is my armory
http://www.wowarmory.com/character-sheet.xml?r=Onyxia&n=Eesti

That potentially answered, to this point I've only OT'd Kara and tanked Heroics. Not having a problem with either and thinking I should be fine MTing Kara...what would my chances be in ZA? What can I improve on?
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Eesti - U.S. Onyxia
http://www.wowarmory.com/character-sheet.xml?r=Onyxia&n=Eesti

The T4 faceguard has 0.5% less uncrushability than the Tankatronics, the Sabatons have 1.22% less uncrushability than the Battlescars, assuming identical sockets. What were you planning on using to make up the difference?

In general terms though, I'd suggest no to the helm swap and yes to the boot swap. The Tankatronics are superior to anything this side of the ZA helm or the T6 helm. I'd only consider the T4 if i was desperate for spelldamage.

As for the boots, sure go for it, the Sabatons did me until I got the Tide-Stompers and I still have them around for my BV set.
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I don't wish to come across as surly or mean-spirited, but I am, so that's how it comes out.
Armory : Gear Advice

Thanks for the advice Ulushnar. Tankadin is showing me 3.23% over uncrushable when HS is up, so I had assumed that it might be ok to drop that down as close to the minimum as possible. I'll make the boot switch when I get the badges. As to the second part, based on my armor (linked above) what would I run into trouble with in ZA (and MTing Kara, for that matter). Thanks in advance for any comments.
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Eesti - U.S. Onyxia
http://www.wowarmory.com/character-sheet.xml?r=Onyxia&n=Eesti

Heroics: No Problem
MT Kara: Fine
ZA: No problem on the first three bosses. Dragonhawk and the big two might cause problems.
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I don't wish to come across as surly or mean-spirited, but I am, so that's how it comes out.
Armory : Gear Advice

My pally gear is similar to yours and I have tanked all of ZA including Hex Lord and Zuljin with no problems.

The only problem I see is that your spell damage is quite low, I have around 420 with similar-level gear.

I would suggest getting belt of the guardian crafted, and try to replace your gloves and shoulders with T4 if possible, for a large threat boost with little loss anywhere else.

Jungle Stompers from Nalorakk and the heroic badge tanking ring would also be good upgrades