Just 2 thoughts.
are your LEDs providing the wavelengths necessary to act as grow lights? My understanding of sunlight, LEDs and the growth process is that 400-700nm is where its at for optimal growth. going above or below, those limits is wasting energy.
my other thought, also regarding efficency of growth, and quality of light is to ask what if you PWM your LED output?
PWM can greatly improve your efficency, but for these purposes, i dont not know how it would effect quality of light pertaining to plant growth. I mean plants dont have retnas, they dont see, but they do response bio-chemicaly to energy levels, which are effected if you use PWM. using PWM LED output may still offer some charming results to efficiency in your application.