I’m not sure I’ve ever seen an example where “efficiency” wasn’t just giving up on social obligations (either by screwing employees, or screwing customers, often both).
I’m looking around and see postal services (it still breaks me that “going postal” means what it means), roads, railways, energy production. I don’t see any of them having gotten meaningfully cheaper or providing better service or handling their employees better through privatization.
As an hybrid, splitting into nationalized infra management and privatized customer facing service potentially worked for ISPs. That’s the only mildly positive example I can come up with.