To my knowledge the DK tank should already give you the increased spell damage debuff? If thats just a specced thing then the lock would increase your damage quite significantly. The other thing that you forgot is the improved shadowbolt debuff for 5% extra crit which is pretty big (again specced).

On the other hand having a different class in your party does give u more things to think about. I dropped a shaman for a balance druid, and while I have grown to love it now it did take me a long time to get used to him, and for a long time i was wishing I had kept with 4 shamans. Having more to think about itsnt neccesarily a bad thing though because it means you have more options. Healthstones, pet offtanks, fear bombs, are all things that could have come in very handy for my party at times