Does FaceID consume a significant amount of battery throughout the day?
No - I tested this for several days with Face ID on and with Face ID off and I can't measure anything even close to 15 minutes or more difference in run times / overall battery difference.

My intuition is that whatever processing power is needed to power the infrared is more than counteracted by saving time unlocking where I'm tapping out my passcode to unlock the phone as opposed to having no passcode.

I disregarded entirely:


not having a passcode / unlock
disabling attention


My passphrase is something I type out with letters and numbers and not a numeric only short passcode, so ymmv but I'd be surprised if you have measurable differences on your device and iOS 11.