I recently purchased the GA Z97 MX Gaming 5 mobo and the Power Color R9 280X TurboDuo gfx card, and no matter what I did I could not get the PC to boot properly.
When the Gigabyte logo eventually made it on screen you could see it being drawn line by line, sometimes it wouldn't even post. So after much faffing about pulling everything out one at a time, checking for bent pins etc etc I eventually got the PC to boot using the on board gfx via the DVI port. So I now know the mobo works and the gfx card was tested in another machine and worked fine.
This allowed me to get the basic drivers installed for the mobo and get the bios flashed to the latest version, which is F4.
I then went through the bios settings to see if there was anything that stood out as problematic. In the end I had to go into classic mode (F2) and then click the ST button. Under miscellaneous there is an option to set the PCI-E ports to either AUTO - Gen1 - Gen2 - Gen3. I changed it from auto to gen1 and got a successful boot via the gfx card. I then tried gen2 which gave the same result. Trying gen3 the computer went back to not posting.
The gfx card is sold as PCI-E 3 so there is clearly a problem somewhere between the gfx card and the mobo communicating at that level. Hopefully posting this here may help someone out if they have the same problem and maybe a bios update in the future will resolve this problem.
Just to note, when updating the Bios using the @BIOS utility, when setting it to update from the Gigabyte servers it only points to the F3 update, to get F4 you will need to download it and update from file.
For the record the system I'm using has an i5 4670k cpu and 8GB (2x4) 1333 ram.