The heaviest lead I,ve used/are using is 6 oz, the Korda gripper lead as it happens.
The heaviest feeder is the Drennan 2 and half ounce oval blockend, into which I push a 4 oz Korda gripper lead, and then fill the remaining space in the feeder with pellet paste. So I imagine that combo weighs getting on for 7 ounces. Usually it manages to stay still for a while, until dislodged by weed/leaves etc building up on the mainline.