Its come to my attention that the Grothdarr monster set is not improving when its quality is upgraded, even though the tooltip description on the item says it is.
I have just upgraded the set from blue to legendary quality, and with my current stats my Grothdarr in legendary quality reads 2520 damage every 1 second, but in actual practice deals 2178 damage, the same damage as before I upgraded it. I have tried equipping more armour pen, or using breach to reduce the enemies armour in case it was being reduced, but neither had any affect on the final damage output.