Did you use the GRUB_CMDLINE_LINUX_DEFAULT="quiet splash pci=nomsi" command? I think that just suppresses any PCI-related error messages… But if it’s made the card work then all good!
Further down that thread there are two other suggestions:
Disable PCI Active State Power Management in your BIOS. Could be called ASPM or PEG-ASPM. Should be easy to test.
Or if you’re more daring try updating your kernel http://www.omgubuntu.co.uk/2017/02/ukuu-easy-way-to-install-mainline-kernel-ubuntu)