> or 2 x 2 Gbps shared
Only if both drives are saturated at the same time. But since the context here is a NAS usage, there's a max 1gbps upstream/downstream in the first place so you could comfortably do a 4 drive mirror NAS off of this and not have any bottlenecks on the USB 3.0 -> SATA side of things.
1 Gbit Ethernet over RGMII
4 Gbit PCIe Gen 2 shared by 2 USB 3.0 (so 1x 4 gbps max or 2 x 2 Gbps shared)
Total: 5 Gbits, no margin to spare