I play a warrior and parsed HGLs vs Black Sapphire Electrum Earrings while leveling in The Hole for hours at a time and hundreds of kills (ie, not an amazing sample size, but not terrible), and experienced about a 4% increase in damage taken when tanking Slab and Docks mobs with the HGLs vs the Black Sapphire earrings and only a 1% damage increase when tanking GT mobs, which average a few levels higher.
Now that I'm 60 I've tried stacking AC while ramp tanking vindi compared to when I'm in full HP mode MT'ing Vindi, and am able to pull off a modest 150ish displayed AC difference. I've found that in RT/AC mode the average hit I take is about 20 less than in MT/AC mode. That said, my sample size here is very small (I've only thought to track this over the course of a few vindis).
Take this for whatever it's worth...which is probably not much
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