Your criticisms are totally valid and some idiots on here ARE trying to shut you down. If you have an M8 and an S6 side by side, and in your experience the former was stutter free and had better battery, then that's as hard evidence as you need. Why would a general article online sway you another way when you have specifically confirmed your experience.
Some people are just hurt you're criticising their dear smartphone. It's silly. It's a piece of metal designed to serve a customer. That's all. Be done with these stupid loyalties. And no it's totally unreasonable to expect a Samsung update to fix these issues. It's an incredibly expensive device. Why take such a chance for an update that may or may not fix the issues? My advice, return it whilst you can and pick it up later if things improve after the supposed updates. That's sensible, not holding on to a potential hole in your pocket. You have to decide whether you can live with the device 'as is' in it's current state, not on a fairytale state in 3 months time.