It's about 75-90% the speed of light, but even that's too slow.
Modern hardware components are getting to latencies in single digit nanoseconds.
Light travels about 30cms in a nanosecond, so extending a pcie port to a different box is going to have a measurable difference.
A single round trip isn't going to register, but there are multiple in a frame, so it's not inconceivable that it could add up at some point. I would like to see it demonstrated, though.
Without one of these rigs, you would not be able to do much at all because of the limited PCIe slots in the host. "not much" here means render times into the hours per clip to even longer. With the external chassis and additional cards, you could achieve enough bandwidth for realtime playback. Specific workflow would have been taking Red RAW camera footage that takes heavy compute to debayer, running whatever color correction on the video, running any additional filters like noise removal, finally writing the output back to something like a ProRes. Without the chassis, not happening, with the chassis you can do realtime playback during the session and faster than realtime during rendering/exporting.
Also, these were vital to systems like the MacPro Trashcan that had 0 PCIe slots. This system was a horrible system, and everyone I know that had one reverted back to their latest 2012 cheese grater systems with the chassis.
There was another guy I know that was building his own 3D render rig for his own home experimental use when those render engines started using GPUs. He built a 220v system that he'd unplug the dryer to use. It had way more GPU cards than he had slots for by using PCIe splitters. Again, these were not used to draw realtime graphics to a screen. They were solely compute nodes for the renderer. He was running circles around the CPU only render farm nodes.
People think that the PCIe lanes are the limiting factor, but again, that's just for getting the GPUs data back to the screen. As compute nodes, you do not need full lanes to get the benefits. But for doubting Thomas types like you, I'm sure my anecdotal isn't worth much
The problem you will encounter with pcie gen5 risers is signal integrity.