Few things...
1) "Def cap" is a terrible term and I hate it. Defense is still an awesome stat even if you are beyond the "cap". The ONLY thing that changes when you hit 440 defense is that any additional defense will not lower your chance to be crit by a boss because you are already at 0%.
Many people make a huge deal of this because a crit hits for 200% (double) normal damage. The thing is, it doesn't matter if you have 440 defense or 300, bosses still have a 15% chance to land a crushing blow which hits for 150% (1.5x) normal damage. Aside from the situation where a boss might 1 shot you on a crit, crushing blows are statistically worse (half the damage increase BUT it happens 3 times more).
The only reason anyone would NEED def cap outside of a couple bosses in BWL is if you are a really bad tank and can't push shield block every 5 seconds.
2) You are massively under valuing armor and stam. Going from pre-raid to T1 is a HUGE upgrade in armor and stam. On top of that, there is enough avoidance (dodge/parry) on T1 pieces to make up for the avoidance you lose with the defense reduction. Then there are the set bonuses which are actually quite nice, especially the 8pc.
3) The only places where I would care at all about def cap are broodlord, nefarian and maybe chromaggus (depends on the breath).
Broodlord uses 2 melee abilities, knockback and mortal strike. These are in addition to his auto attacks which means you can't rely on shield block to push crits off the attack table. If you are crit with a mortal strike you are in for a bad time because not only are you low on HP, your incoming healing is also greatly reduced. I wouldn't worry too much about being a bit below the def cap on broodlord. If you gain 1,500 HP and 1,000 armor but drop from 440 to 415 defense, I would say that is definitely worth it on broodlord.
For Nef, when he uses a warrior class call you are stuck in zerker stance and can't use shield block. So for nef it is nice to be def capped but again, I'd rather have the stam/armor of T1 and be slightly under the cap.
On Chromagus you will get stunned often and can't use shield block while stunned so def cap is kinda nice but most of chromag's big damage are from nasty breath attacks and defense does absolutely nothing for that.
4) These are the only pieces I would consider using over T1.
http://db.vanillagaming.org/?item=19894http://db.vanillagaming.org/?item=14552http://db.vanillagaming.org/?itemset=444 (If you have 300 blacksmithing, T1 is still SLIGHTLY better for mitigation but 4% hit is amazing).
5) By the time you are on bosses where it really matters (AQ and Naxx) you will have 425-440+ defense just by gearing BiS with def cap ignored. At that point, it's easy to just shuffle a couple pieces of gear around and be at the cap when needed.
tl;dr: Def cap is massively over rated.