I'm not sure what your current question is but (ignoring the GT/s PCIe spec for a moment) each PCIe 3.0 lane gets you:
- approx 1GB/s - which roughly equates to a saturated 10Gbps ethernet link
With 4 lanes available to you you still have around another 3GB/s going spare even if the 10GbE is maxed-out, plenty for a NVMe read/write cache.
In reality you will not be producing and consuming that amount of bandwidth simultaneously on an 8-bay NAS as the only high-bandwidth way in or out of the NAS itself is that 10GbE link. This leaves plenty PCIe resources available for internal NVMe / SATA cache management, even if we pretended that the other system bottlenecks didn't exist.