One thing I forgot that keyclone mentioned that helped me IMMENSELY was the maxfps/maxfpbk setting. I manually set mine to /console maxfpsbk 20 normally but when I went to 5boxing, turning it down to 15 on all windows at once made a HUUUGE noticeable improvement from barely 20 on main while alts had to to 15capped on alts and near-max fps on main window again.
/console maxfps 60
/console maxfpsbk 15
???
Profit! ;p

As for Mages, they have alot of nice instants(nothing like multiple pom/pyros) but they're VERY squishy and usually easily killed in pvp and won't be tanking or kiting much in pve. They can kill well enough though