Tank damage during Ragnaros 10m (normal)
Sign up for world of logs. Use one of the uploaders (I like the realtime one). A link to logs would help.
Any tank who says a mastery trinket (mirror) "messes up their trinkets" doesn't know his spec well enough. Mastery is a great stat for any tank. For block tanks, it is unbeatable as a stat. For dks and bears, there may be some balance to be had with other stats but it is still near best.
Given that information alone, even without an armory link, my guess is gearing/gemming/reforging/enchanting is an issue on the tanks. I will bet they are chasing item levels instead of optimizing stats. It sounds as if they might even be stam stacking which went out the window with Cata.
For example, for the warrior the wildhammer rep boots are netter than any of the normal mode drops. The crafted are a clear upgrade but not obsidium stompers. The 365 cloak from the rescueing Thrall questline is better than any of the 378 cloaks. The 359 crafted or jumbotron belt are better than the 378 rep, but not the shannox belt.
For the dk, I am less sure about each of those. Generally, two tanking stats (dodge/parry/mastery) is better than one (dodge+hit/exp etc). For a dk, there is some survival value in having some expertise to land more death strikes.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng