7
Had this staff since level 52. I'm at level 65 now and I have yet to replace it. A few have come close, but the +43% damage to stunned enemies as a lightning sorc is really quite incredible. I've upgraded it three (out of 5) times now -- Even Unique Sacred items haven't come close.
Was this just a super lucky drop?
Yeah, but I generally think the 2x benefit is specifically to make up for the fact that not having an off-hand is so limiting in terms of affixes and aspects. Generally, I salvage or sell all staffs as a sorcerer because even the slower base attack speed alone is enough to make me not want to use it.