Hi Vincent,
We have also seen several cameras that are not able to reach 4K@30fps with 2 MIPI CSI Lanes. I think it depends on the sensor, if it supports that mode, the internal clock speed, the MIPI CSI speed on the lanes, etc. In theory Jetson NX (where the driver was tested) is able to reach 2.5Gbps on each lane. The IMX477 uses 10bpp so it should be possible to get 33fps.
This post is for Jetson Nano questions, we are currently working to port the driver to Jetson Nano and run the test. However, Jetson Nano is 1.5Gbps per lane. So the max fps should be 22fps. You can use this RidgeRun Spreadsheet to get some theoretical numbers. You can use the Jetson NX post if you have questions about the usage of the camera in the NX board
-David