Indeed, the Armaghedon and Nightmare rounds are pretty broken, but reducing the health of snipers and survivors to 1.25k is not a solution in my opinion. Then you would create imbalance for them, and it would be too easy for the zombies to win. Basically a survivor and a sniper would be dead in 5 hits, which means insta death against 2 zombies.
That's why i agree with TreY, but i think even Survivors should have 5k HP. If you are against a few survivors with Jetpacks, it's impossible to kill them at 12k 🙂. Reducing the health only for Snipers would balance the Nightmare round, but it would have no effect on the Armaghedon, since we have no Snipers.