I submitted a bug report to AMD as follows, if others can do the same it'd be appreciated:
This problem applies to catalyst drivers starting from the RX580 Press Release drivers and is still current as of 17.4.4.
The new drivers which support the RX 580 now cause a BSOD with message "ATTEMPTED_EXECUTE_OF_NOEXECUTE_MEMORY" when attempting to use greater than 5 GPUs. This was not the case prior to the RX 580 Press Release drivers.
The testbed is a Z270-A Pro motherboard with a Celeron processor, 8GB RAM, 1800W PSU, using Sapphire Nitro+ RX 580 Limited Edition graphics cards. This configuration works for RX 480 graphics cards using sub-17.3.x drivers with 6 GPUs under Windows 10. Further, the same system supports 7 GPUs under Ubuntu using amdgpu-pro drivers.
Isolating the problem consisted of having 5 working GPUs and attempting to add the 6th. On boot, BSOD noexecute error. Unplugging ANY of the existing cards alleviates this BSOD on boot. Further, once 5 GPUs are active, you can disable one of the GPUs and install the 6th GPU without error as seen in the attached screenshot. The moment you select to re-enable the disabled card, again BSOD noexecute error, seemingly confirming its a driver issue. The choice of disabled card is arbitrary, it works with any one of the cards disabled. The hardware works as confirmed by booting into Ubuntu and all GPUs are operational.