You are right to be concerned but here are a few of the critical things that make the diference:
-HPS output is concentrated in the green/yellow/orange range which results in more lumens/W, but less photons/W than 3K LED
-The light is more evenly spread in the canopy, all the way to the edges, this is huge
-As you said, less lens/reflector losses than HPS hoods
-Finally, DIY COB grow lamps can now have much higher output efficiency than HPS.
And a few speculative reasons:
-The LED spectrum is more evenly spread across the visible range, probably increasing photosynthetic efficiency
-The LED has more output in the red range, probably increasing photosynthetic efficiency to some extent
The design you proposed would be about 51.3% efficient. 657 dissipation W would equal 337 PAR W. The 1000 HPS emits 360 PAR W. Your design would strongly outperform the 1000W HPS. Many growers are telling tales of 1.3 gpw with the new 1000w DE bulbs. If that is true, they should be achieving much higher gpw with CXBs (unless it turns out that radiant infrared light grows bud!) With organic soil, I get between 1.0 and 1.4 gpw with CXAs at 47% efficiency.
