Just replying by the seat of my pants, I'd say it's OK to stay with 12mm, though I'd really recommend half inch
.
It seems all but impossible to me to load 6,000 lbs of cruising gear onto a 44-foot boat. And even if you did, how often would would the rig be loaded up long enough for creep to occur? Unless creep occurs while the mast is at rest (which is what "sizing for creep" is supposed to prevent), it'll be a long time before you strain the rig for long enough that creep will be noticeable.
That's just the opinion of a notoriously unscientific guess-timator. If one of the resident math wizards tells you otherwise, go with them.
Ben