I agree that a powerfist is necessary for a unit of DC. But beyond that, I do not add anything (Other than possibly a chaplain!). Power weapons are a *large* point sink. If you think about what you would be applying the power weapons against, typically they are units with small numbers. Your weight of numbers with 4 attacks on the charge at STR and I 5 should be able to drag down quite a few without needing to spend the extra points. Consider spending the points from power weapons on a few more members.
Put them in a storm raven, and let them assault out of it where you need them. DC are useful for killing one enemy unit. After that, it's about positioning with consolidation after combat and luck.
"War is an ugly thing, but not the ugliest of things. The decayed and degraded state of moral and patriotic feeling which thinks that nothing is worth war is much worse. The person who has nothing for which he is willing to fight, nothing which is more important than his own safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." John Stuart Mill