Yes it can boot with NVMe M.2 ssd using a PCIe adaptor, it will work at 4x speeds.
GPU will be at 8X but you won't notice difference there.
Support for NVMe boot may require bios update.
ASUS TUF Sabertooth X99 already supports 2.5" and PCI Express card NVM Express options. Now ASUS is world's first motherboard brand to offer the same support across the entire range of its consumer Z97/X99 motherboards. By simply updating the UEFI BIOS and using an optional ASUS Hyper Kit expansion card, your motherboard is upgraded to achieve more than previously possible. It is possible to be faster and use less power; extreme data-transfer speeds up to 32Gbit/s, while better power-efficiency leads to lower running costs.