Without reservation I believe Hard Shot is more effective in killing birds than is soft shot.
I believe this because research has shown it as demonstrated in Bob Brister's Art and Science of Shotgunning, Oberfel and Thompson's Shotgun Patterns, to name a few.
As earlier posters have pointed out Hard Shot which holds it shape and flies truer than softer shot which does not yeilds better patterns, velocity, and hence effects on target.
Plating if it is linked to actually harder shot does provide better performance to include less "feather draw" which is what I call the downy feather pulled into the wound by rough soft shot which disappears with quality plated shot in my experience.
