I'm using WhatsApp and I haven't missed any messages yet. I'm not a heavy user but I have had some pop up while the Cosmo is closed/locked.
I do have DuraSpeed disabled, but I haven't set this app to no-optimize (yet).
If I go to Settings : Apps : WhatsApp : Battery, mine is currently set to:
* Background restriction : App can use battery in background
* Battery optimization : Optimizing battery use