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.
I use the 6 oz leads to hold bottom where the barbel are, that,s all. It,s either use them and catch, or use lighter leads and go home fishless, with my bait and end tackle constantly heading for Chepstow !