I'm thinking of fabricating my own custom steam box case that has a similar layout to the beta version that Valve sent out. A flexible PCIe riser seems like a good solution for making the GPU be horizontal in the case. It would also help eliminate problems with accuracy in the PCI slot cutout locations.
But does anyone know if this riser will impact my gaming performance in any way?
The throughput will be the same, but people have noticed problems with the cheaper flexible risers due to interference. You can wrap it to protect this if you notice anything, but there wouldn't be any material gaming performance change.