If you're on Virgin 152 then set it for 160. You're trying to over engineer 152mb into 18MB but the Asus router deals in mb not MB.
Are you streaming netflix and playing wow on the same machine? I seem to remember you can prioritise by IP also on the Asus? If you are using different machines, try assigning priority by IP.
Also if you can't go by IP, try changing your rules to tcp/udp rather than just tcp. I don't know wow that well but if it is using UDP then that could be being seconded to medium priority tcp traffic.