admittedly they do spec the SST-40, so in an ideal world 183 lumen/watt, but they also do state 92% efficiency.
1600/183=8.74w @ 92%=9.5w
24wh vs 40wh if we believe that 92% efficiency claim, that they emitter does not get hotter than 85c due to heat sinking, and that they're using the top...