So I've gotta ask, does not having these packs leave the Necrons underpowered?
Not appreciably. You're missing out on a warlord, obviously, but he's arguably not as good as the two warlords in the core set. You'll have slightly fewer deckbuilding options with Necrons, but since they have access to practically the entire unit cardpool (if you include out-of-faction cards) that's not really going to reduce your options all that much. I took a quick gander at the Necron cards in Deathworld, and honestly this is the only one that I'd miss:
I suppose if you can find a copy of Jungles of Nectavus (contains Shroud Cruiser) or Searching for Truth (contains the third warlord) at a decent price they might be worth picking up if you really like Necrons, but otherwise you're honestly not missing out on much at all.