DPS is important in certain situations. Best I have is 19.5k on heroic Bael without haste buff. In 10m tank dps can be crucial such as P3 of Raggy(wings str pot inq pew pew 24k peak damage). In 5 mans the only thing I do is beat people to make them feel bad.
since your just stepping into raiding there is only one fight in firelands where your personal dps will be an issue and thats Alyrazor,
but since the nerfs even thats not an issue.
if your staying alive + positioning correctly + holding threat + not being a healers mana sponge
= win the only thing i can guess the DK could have potentialy complained about was your positioning as if you had the boss so dps couldn't get behind without being in green goo then thats a reason but it's more his fault for standing it in.
if the healer isn't going oom quick then you did nothin wrong for a 5 man.
Your a tank if you wanted to worry about dps then you should be dps. As long as you hold aggro I wouldn't worry. What would be the pooint in the 3 dps then?