At 55 there might be a small spell balance deficiency that is the cause.

I am probably in the minority, but I would roll full affliction + disp and just go for fast dot application mostly relying on instants, minus the opener. From memory without jamba/isboxer targets this is what i roll with from the 'apply macro'. The deathcoils are there to keep things from wrapping.

PHP Code:
/cast [nocombat, harm, nodead] Unstable Affliction, Death Coil
/castsequence [combat, harm, nodead] reset=target/combat Bane of Agony, Corruption, Death Coil, Death Coil, Death Coil 
/cast [harm, nodead] Shadow Bite

/cast [harm, nodead] Smite 
I find myself draining for a nuke, not because of efficency but because of the instant kill ... zero overkill and fast mob turn over.