"As I hear it, a large coal powered generation plant in the USA runs around
85% to 90% efficient."

A large coal fired plant runs at about 30 to 35% thermal efficiency.

Then you have to deduct a further 7 or 8% losses overall in the transmission
network.

A ton of coal 2240 lbs (at 13,000 btu/lb) will produce about 2800 kWh at the
power plant, or about 2600kWh at the socket. Assume that it takes nearly
1lb of coal to make 1kW of electricity.

This is a figure that has changed very little since the 1920s.

As the calorific value of the remaining coal stock declines (anthracite
14,500 btu/lb, through the bituminous and semi-bituminous grades down to
lignite - brown coal at about 8500 btu/lb) you will have to supply even
more coal to maintain the same electrical power output.

Gas turbine power plants are a little better at about 45 to 48% efficient.
Some under devlopment might just exceed 50%.