I assume you are in full underworld since you are doing NiM. As has been mentioned in this thread, you should not need to change your tanking stats for power etc. Just work on your rotation. I did Kephess NiM yesterday and was close to 1.3k DPS every time.
Using less Sundering Assault and more often Vicious Slash is a realy good hint. It is impossible to test my rotation on a dummy because of low rage/focus.
Im playing the same rotation as you, with one important change, that i have Vicious Slash in it. I will suspend it from my rotation.