Hard Capped at -50% for PDT and MDT. Defending ring in that case would only be giving you -5% PDT and -10% MDT/BDT, but you can change your gearset around and have minimal gains elsewhere depending on slots.

Most likely breath damage is capped at -50% as well, but I know of no one who specifically gears for it, and just wears pieces with -damage on them.