Hey man. Now, i hope nobody will rip my head off for posting here without being a super specialist.. heh
I'm not sure i understand your question correctly, 'cause it sounds like you think the CXB's have LOWER efficiency than HPS... They are much much MORE efficient than HPS, especially if driven at low currents.
To my understanding, some of the best available HPS setups are around 40-45% efficient when everything is brand new. CXB 3590 driven @ 23watts each are about 64% efficient! ...and they will stay that efficient for years if cooled and maintained properly. @50 watts they're 54ish% efficient. Check the COB Efficiency Spreadsheets thread, very useful info there.