Yes, Lackey Sliver is retarded. Lackey is bad enough with Gobbos, and doesn't give its ability to every other Goblin. Slivers can get much much dumber than gobbos with a large board presence, and the Big 3 would be really really retarded with this.
Wither seems really Meh while infect exists. That said,

would be too light for the infect sliver.