There is a fair amount of science behind growing with led's. From what ive learned the keys are Spectrum, Wattage, and Penetration. As the technology is evolving the led's themselves are becoming better tuned to the spectrum's needed, More powerful and efficient which also plays into penetration. Current led technology already surpasses antiquated HID lighting by producing Far more Usable light per watt of energy used. As led technology progresses I expect we will see manufacturers working harder to manufacture the Exact Spectrum's needed by plants. Good quality led grow lights already produce pretty close to the ideal spectrum's, But I imagine we will see even more exact spectrum led's in the future. For example early led lights used only generic red+blue spectrum's as this is what was believed to be the answer. As time has passed and led grow lights are being tested more thoroughly by both experts and growers its becoming obvious that a small amount of "white" light is beneficial. As such most manufacturers incorporate a small amount of "white" led's. Another advancement has been more targeted spectrum's, For example original blue led's were approx. 460-470nm. Most quality companies now use led's more in the 445-450nm range which is a far more effective "shade" of blue light for growing. Basically I think we will see more small improvements in overall efficiency in the years to come. But in my honest opinion led grow lights are already plenty efficient to replace HID.
Many growers have already made the switch to LED and are Not looking back, Myself included.