Gamers are easy "marks" it seems. IMHO this is all crap and these devices should be avoided if this mis-feature can not be disabled.
EF will still end up AC_VO and hence above AC_BE and AC_BK, so EF still does the right thing. I also am not a big fan of WMM, but that is a different matter, in short I believe using CS6 for nor critical traffic to be not optimal, but hey, everybody needs to set the policy for their local networks and as long as that is consistent there is no better or worse IMHO.
As always, as long as AC_VI stays empty AC_VO will effectively be the one with highest priority and things will just work...
I disagree, the highest one actually in use (or put differently one class higher than the rest seems sufficient), that way one can extend the network to deal with more important packets in the future, turning to 11 from the beginning is too "spinal tap" for my taste
But again this is a matter of taste, and there is no real right or wrong, except that even a nice ISP will bleach CS7 and CS6 on ingress, while EF might have a chance to survive the internet passage, which might be interesting assuming one trusts the sending parties marking...