There aren't caps per se on any stat, we are just limited based on equipment available (the equipment itself does have stat caps based on item level). Most important stat for tanks is hitting the accuracy cap with the best gear you have available. The main job of tanks is holding the hate, and if you are missing it will affect your hate generation. Beyond that, focus on Parry and Determination, then Crit/Skill Speed if Det or Parry is not available.

Accuracy req'd as follows:

Lv50 Dungeons, Hard mode primals, Garuda/Titan/Ifrit Extreme
Minimum ~430 accuracy

Coil turns 1-4
~476 accuracy

Coil turn 5, Levi/Mog/Ramuh Extreme
~486 accuracy

Second Coil turns 1-2
~495 accuracy

Second Coil turns 3-4
~515 accuracy

I think that's right anyway, someone will correct if not I'm sure.