Hi, sorry for the late reply,
the show notifications option only controls whether the app is allowed to show an icon in your status bar at the top of the phone. So it's unlikely that this will effect the battery life, but give it a go anyhow, and let us know how it goes.
yes, when you disable apps the Galaxy S4 will ask if you want to replace it with the factory version, this is simply because Samsung does not allow you to completely remove the app from your device.
If you select disable, this is what will happen:
It will ask you to replace it with the factory version, this basically will remove all updates to that app, putting the app back to its default version and then it will be disabled.
If you ever want to enable it again, it's very simple. You just scroll to the very bottom of that list where you will find all the disabled apps. Just change it back to enabled, next time you visit the google play store it will show that the app has updates available. click to update the app and it'll be all back to normal again.
Keeping this in mind, you can disable apps without worrying about deleting them, and any apps can be restored very easily.
Hope this helps.
EaziG