I do have iperf3 testing already setup on a stable host server pair in my network... so
:::

Setup two vlans A, B. These vlans have no dhcp, no routes, nothing - Attach a interface to each vlan, assign as static ip address on the same subnet, run iperf3 in server mode. The test comes from how you bridge the vlans: A switch with vlan A on port A, and vlan B on port B, now if you plug in SFP.A and SFP.B and bridge with a cable... iperf3 can connect.

This extends your in place network path from interface A to interface B by forcing it to go over SW.PortA->SFP.A->Test Cable->SFP.B->->SW.PortB

Host interface A
  │
  │ VLAN A
  ▼
switch.portA ── Test SFP.A
            │
            │ test-cable
            │
switch.portB ── Test SFP.B
  ▲
  │ VLAN B
  │
Host interface B

Run iperf3 in a loop, and now you can swap modules/cables and get the full benefit of a point to point cable test without having to move any machines around. This will even work with a single machine for testing, with a interface on A and B (different namespaces), you just have to double your iperf results mentally.

I realize this is fairly niche, and probably obvious to many people, but I just stumbled upon it today and thought it was pretty neat.

FWIW my SFP modules ran at 9.6gbps, so I'm happy with them!