Yup. BumbleBee Wiki is very helpful. Do NOT download and install nVidia like you are trying!
I followed those instructions setting up my new laptop with Nvidia Optimus and never ran into an issue with anything ever wanting a nouveau driver. At any point in your installation do you install nouvea? You never need to do that as far as I know. Sorry I can't be more helpful.
Honestly, glxgears is not a true benchmark. But the discrepency is suprising. Have you went through the Wiki (https://wiki.archlinux.org/index.php/Bumblebee) and checked if there is any troubleshooting steps? Also, how are you executing glxgears to test your results?
Unless I'm mistaken, won't the GTX850 use the latest nvidia 343 drivers? The nVidia website shows both 340 AND the 343 BETA for that card. I only have a GT 730M on this laptop and am running the nvidia 343 drivers.
I would start here if I were you:https://wiki.archlinux.org/index.php/BumblebeeUnfortunately, Arch requires some knowledge of Linux, etc. to get the most out of it. Obviously, feel free to ask questions, but a couple of your questions are answer in the Wiki(s) linked.