I would get some more basic infantry. The DA have great, cheaper veteran squads. I would take a full rhino with either a veteran squad or a tactical squad to add some flexibility to your army. Also try to put a company banner in with one of your deathwing squads. I do agree with mixing them up and the 4+ SS are garbage. One last thing about Belial, run him with dual lightning claws. He gets an extra attack (better then rerolling one failed to wound) and then rerolls all those failed to wounds.
Everything I've ever done comes down to the roll of this one dice... and it was a 1