I have been testing Mercs non stop since release and I thought we could share hard data from the real world. Without further ado.
Level 80 Tank Merc Journeyman Tier 1
Level 70 Healer Merc Apprentice Tier 5
These two with DPS from an 80 Necromancer and a 66 Air pet tanked and healed Komodokin in Sunderock. He hits for 4000+ so I was surprised they could handle it.
The same Tank merc could tank the real 7th Hammer PoP raid mob but DPS from Mage and I could only get him to 70% before the Cleric merc went OOM. 7th Hammer has 1 million HP roughly and hits for 1500 with a few 800-1000 AEs.
Going to try SoD zones with the 80 Tank merc and will post results later.