Well, to fix my earlier statement, Screamers go down fairly easily. A couple of bullets or a well-placed bap with a club will take down a screamer immediately. Demolishers.. not so much.
As for the Behemoth, I was incorrectly assuming it had much more HP than a Demolisher. If clipping was an issue, could they not have worked on the issues to fix it? Even if they had to set it aside for a while. It's not as if we don't already have clipping issues with existing zombies.