I can confirm that the OS completely hangs when unbinding R9 380 (Tonga Pro)
with X running. Works fine with X off.
Thought I'd add my post from the linked thread, so I can be updated.
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I have amdgpu and vfio-pci both in kernel, used the following to unbind it.

#!/bin/bash
for dev in "$@"; do
        vendor=$(cat /sys/bus/pci/devices/$dev/vendor)
        device=$(cat /sys/bus/pci/devices/$dev/device)
        if [ -e /sys/bus/pci/devices/$dev/driver ]; then
                echo $dev > /sys/bus/pci/devices/$dev/driver/unbind
        fi
        echo $vendor $device > /sys/bus/pci/drivers/vfio-pci/new_id
done

lspci -nnk shows:

03:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc.
[AMD/ATI] Tonga PRO [Radeon R9 285/380] [1002:6939] (rev f1)
        Subsystem: PC Partner Limited / Sapphire Technology Radeon R9 380 Nitro
4G D5 [174b:e308]
        Kernel driver in use: vfio-pci
03:00.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Tonga HDMI
Audio [Radeon R9 285/380] [1002:aad8]
        Subsystem: PC Partner Limited / Sapphire Technology Radeon R9 285/380
HDMI Audio [174b:aad8]
        Kernel driver in use: vfio-pci
        Kernel modules: snd_hda_intel