At that level of damage per hour, and I have read of those who claim much more, I bet that you need the ultra hard mode being introduced as it must be of little challenge at times.
My whole gaming time is spent in Benny Hill mode...! Kinda fun as there is a real risk of me dying and only doing 200 or so Max damage.
I could mod the weapons for more damage but tbh like you said zombies don't last long. One thing I do think people need to use more is camouflage, it's incredibly good on missions, couple it with the takedown skill via sneak and you can basically walk around snapping zombie necks without being attacked.
I'm playing it through again with a friend in coop, it's pretty hilarious being high level back when the low level zombies were barely aggressive even at night, like a hot knife through butter!