I have been having the same issues myself as noted above but since I do not have anything new to add to the thread I did feel the need to post until today.
Instead I have been making little adjustments to help improve performance but nothing helped until yesterday when I enabled NCQ support.
I am now able to stream movies from Plex while performing other read/write operations on the array such as running backups etc without any hiccups. Performance is still not up to par but it has helped allot, not sure why it would though.
Not saying this is a fix that will work for everyone because I am sure that hardware support would be an important factor but I may be worth a try.