Paul,Hi Graeme, by focused energy I assume you mean a reasonably straight fly leg aimed at target with a starting position of a fully straight line and a correctly aligned backcast?
Why would the kinetic energy of a nominally straight length of line traveling at some velocity be any different than the KE of a curved length of line traveling at the same speed along its curved path?
Because of loop morph, I would think the KE in the y direction in the curved length of line would not be lost to the lost to the cast. Although the added form drag in the curved line would be higher than the drag losses in a horizontal line.
I agree with Bernd that there are no straight lines in casting and have yet to see a long fly leg that is perfectly horizontal so that all of its KE would be confined in the x direction.
Focused (i.e., confined to one direction) or not, energy is energy.