I guess I just don't see a 45 second cooldown on a pet as a great design for a spec's primary aoe element.
One thing I did fail to consider is that all those Demonwraths generating soul shards for HoGs will end up summoning a lot of imps. So you can get off a bunch of implosions over and over. So, you can throw that in the mix as well. Plus life tapping because Demonwrath will suck your mana dry. It's just a lot of micro-management, especially compared to most classes who's aoe output consists essentially of 'press my aoe button'.
I do have a question about Implosion. Is that 100% spell power damage per imp? Not in total? I know Demonwrath's damage isn't affected by the number of demons involved so maybe Implosion isn't either? If so...that's a very weak talent. In an aoe situation what would be the reason to use HoG with more than 1 shard if 1 imp blowing up does the same as 4?