Thanks for sharing. My own experience actually reflects that. A templar in epic with maxed DR TR and +3 or 4 all skills, ~180% physical damage collected on gear and 700 strength. DR stops being efficient - and by being efficient I mean oneshotting mobs, why would I need an expensive skill on long cooldown that doesn't. Like it oneshots tigermen but not draconians, so I'd expect it won't oneshot anyone in legendary at all. Not using any debuffs - templars have no aoe debuff skills. And using weapon debuff defeats the whole purpose - I wanted DR to kill packs with one hotkey instead of Lclicking each mob.
Maybe it can work with some mad gears setup like a harbinger dual wielding Giant's tooth mace, not something I want to level though.
320 physical damage is like a single autoattack, its not low but not really enough too. Compared to ~300 vitality and 700 electric burn in TR you can get if investing in int instead of str.