8.5W at idle is high. Are you running power-profiles-daemon or powertop --auto-tune?
I was able to hit 4.5W on my FW16 doing what I mentioned above, but on Gentoo instead of Arch Linux. I’m not sure where the difference is, I’m still tracking that down. Hopefully it’s not all savings from compiler optimization. On Arch the lowest I get is around 5.5W.