Yes 20Gbit/s is fast enough for 4K DPX if half the bandwidth is left available by other devices or R3D regardless of the devices attached. The problem is more to do with latency than bandwidth with other devices chained. Every device added to the controller increases latency. When you add that latency to other latency points especially with the GPU acceleration pipeline then realtime applications such as editors and compositors will have issues if the total latency for the pipeline exceeds what the player can handle. This is where the application troubleshooting is going to get very complicated very quickly.
Eric-ADK
Tech Manager
support@adkvideoediting.com