I just tested it on my priest, 11171 DA shield. o.O
http://www.worldoflogs.com/reports/j...?s=2409&e=2829
Log report for Domo kill last night you see DA averaging 8.2k with 82% uptime. I ended the fight fairly well in terms of mana, was a little diecy for a moment cause we didn't have our normal 3rd healer but it was fine. Only 25% overhealing which for that fight isn't bad.
So yes it does happen, and it happens very often. Maybe you just never noticed the difference or maybe there's some other difference between gear/spec/or some other variable. *shrugs*