Personaly, I would swap out the +def/+<other stat> gems for straight +def gems. Not sure if the armory is not showing this, but make sure all of your equipment that can be enchanted, is. You can also build up a good amount of def from enchants. Remeber, 535 for heroics and 540 for raids are the most important numbers to reach. If you have to dump a little bit of stam to get to those numbers, then you should.
P.S. There is no def cap. You still gain benafit from def above 540! 535/540 are just the minimums you should have to make you crit immune while tanking heroics/raids.