You people who have to pay much for electricity, have you tried extreme undervolting these cards? I tried this briefly on a single r9 290 setup, pulling just 229 watts from wall under mining (55 watts idle). The performance hit wasn't too bad (something like 30% decrease of performance for 100% decrease in power consumption):

http://i125.photobucket.com/albums/p51/Judgment89/7272_zps59b1382f.jpg