Try the 14.6 RC or Beta drivers and see if it's still an issue? I haven't messed with the 14.7s at all, but if I can remember this thread tomorrow, I'll hook up the one 290 I have left with a 290X, and try these drivers, see if I have the same issue and report back (can't do it today cuz I have to sleep in like... 15 mins for work in 9hrs & some change).
But yeah, try the 14.6 Beta or RC drivers. They should still be findable for download on Guru3d.com here: http://www.guru3d.com/files-details/amd-catalyst-14-6-rc2-(14-200-1004-june-20)-download.html ~ Those are the exact drivers I've run in the past with a 290+290X setup and they ran it fine with full Crossfire functionality, tested in Watch_Dogs... hope those help!
CPU: i7 3930k @ 4.6GHz 1.29V (Corsair H110-cooled) ~ Motherboard: Asus RoG Rampage IV Extreme
RAM: G.Skill Sniper Gaming Series (4x4GB 2133MHz, CL9) ~ PSU: Corsair AX 1200W ~ Case: Enermax Fulmo GT
GPUs: 3x HIS R9 290X Reference (Core) Editions in TriFire, Max Load Temps: 71C/70C/71C during OC testing :-P