From what I can understand, the drivers are equally efficient, but the 10's in the HS410 have more excursion, and therefore move more air, and are therefore louder (at least in the lows). Bottom line is, the HS410 and HS210 are designed to get the most from lower powered heads, but still be able to handle higher power.
I had the chance to hear the HS410 and HS210 for the first time last Friday down at Bass Emporium. OMFG!!! I had some idea of what to expect, but I was extremely blown away by the HS410 (and the HS210). We were testing them with an EBS HD350, which puts 350w into a 4 ohm load, and it was loud, full, clear, and just so darned 'good' sounding. Very impressive.
Of course, I still love that NV215. Tough call, there.
Tom.