PCIEx16 drops to x 8 when M2 SSD is installed???

Gen 4 is faster than Gen 3

Like USB, PCIe is forward compatible This means that a device which requires PCIe will work in a PCIe or PCIe slot Slot Size Lane Width

m2 nvme slot With this AMPCOM NVMe SSD PCI-E X4 Adapter, you could add a NVME SSD on your motherboard without slot With the channel of PCI-E X4,

m2 expansion slot 2 #2 +#3 so if use pcie x8 for a 2nd GPU or a card adapter