Use ～に至っては in written or formal spoken reporting when you single out the most extreme member of a series you have just described — the worst-performing branch, the most delinquent client, the biggest loss. It presupposes that a general trend or list has already been stated, then escalates to one conspicuous case, usually with a negative or surprising evaluation. In meetings and written reports it is a step above ～は or ～の場合は in gravity, and sounds out of place in casual conversation.