Here's the data sheet for the HLG-150H:
http://www.mouser.com/ds/2/260/HLG-150H-SPEC-806071.pdf
Bottom right corner of page 6 has the EFFICIENCY vs LOAD graph.
70 watts used of 150 watt max = 46.7% Load.
Looks to be just under 90% efficiency, a drop from your 92.5% on the 120 watt driver, but a gain in flexibility to add more in the future. It's only 2 watts wasted (10% of 70w vs 7.5% of 70w).
