I am actually in the exact same boat if I can jump on this (unfortunately with no solution)....I have a D5400XS board and just bought a Gigabyte HD 7950 to replace a EVGA 275 GTX but get stuck at the 5A error as well. I have updated to the latest BIOS and have even gone so far as to unplug all the SATA devices and reset the CMOS and reset the BIOS and it still stalls at the F2 Splash screen with the 5A error. When I put the old card back into the PCI-E slot it works no problem.
We have lost of reports for these video cards not being recognized properly. Currently under the security tab there is an option called expansion card text, try enabling it.
If the problem continues, this may be an issue with the requirements of the video card as these motherboards do not support PCI Express* 16x revision 3.