They are very different skis. The Hellbents are much heavier skis with a gigantic turn radius. They are anything but nimble. They feel sluggish in tight areas, but you are glad you have them on your feet when coming down off a huge jump into some deep or sketchy snow. The Opus is much more nimble due to its thinner waste, tight turn radius, and long sidecut. They will ski variable terrain with much less effort and leave you less tired, but you wont get that indestructible tank feeling as much like you do when landing cliffs and jumps on the Hellbents.
It really just depends what you are looking for in a ski. Sizing down in Hellbents will help make them feel more nimble, but they are still not like the Opus.