My multiboxing tank is also major raid tank in my guild, 10 and 25 man.

I am currently the best geared tank on my server (5892 GS, 3200 Wow-heroes score) according to Wow-heroes.com (Sethra, linked in the signature).

When full raid buffed (and I sub in a stamina trinket for the nonstamina one) I can hit almost 58k. When I'm 5-boxing, I do 5-mans at 52k-ish.