Just seen this on tech powerup which even futher supports what Tom said in his video about the similarities between the 7970 and 280x, i.e. The two cards can be crossfired together they are that similar.
http://www.hardware-360.com/amd-radeon-r9-280x-and-radeon-7970-work-in-crossfire/
http://www.hardware-360.com/amd-radeon-r9-280x-and-radeon-7970-work-in-crossfire/