Sheepy,
A lot of the difference in glide with a windmilling prop can be attributed to turbine vs piston. To turn over a piston engine that isn't running requires effort to compress the air on the compression stroke. A turbine engine like the PT6 is a lot easier to turn over because the free turbine does not compress the air in the engine.
The energy to turn the engine has to come from somewhere and it comes from the potential energy of the aircraft through a higher sink rate.