If you have not tried Steams Fsync patch, I highly recommend giving it a try. I have been playing Greedfall and under the stock kernel it runs at about half the frame rate as Windows , with the patch it run just as fast or faster than windows. if you are running Arch, here is a link to the steam repo with the arch kernels and aco driver packages.
http://repo.steampowered.com/arch/valveaur/
I'll have to try that. Thanks for the info!