I just started and have a few of the MSI 280x but have not jumped in on this myself, yet. This blog has details for undervolting the card... http://rumorscity.com/2013/12/20/radeon-r9-280x-litecoin-mining-and-sweet-spot-for-700khashsec-part-2/

Also, you can try Trixx. Some of the MSI 280x can be undervolted using that tool (mine cannot.) https://www.sapphireselectclub.com/ssc/TriXX/ Nevermind, saw you're using Linux.