For anyone who might still be stuck, here's how I enabled GPU passthrough using the GTX 670 on my system with UEFI.
Enable IOMMU by adding intel_iommu=on iommu=pt video=efifb:off to /etc/default/grub's GRUB_CMDLINE_LINUX_DEFAULT and running update-grub
Enable the VFIO kernel modules: cat >...