As you can read in this thread in the K2 forum, I created some modified firmwares for the K2. These firmwares offer a higher bitrate for video encoding and minimizes the compression artifacts, which can happen when driving in areas with much details (e.g tree-lined roads).
Now the user @andrew.w.farrar asked me if you could do the same for the C5. Well...of course. So I modified the latest V-42 firmware for the C5 to increase the bitrate at 1080P to 20MBit (default is 14MBit). As I do not own a C5, I am not able to do tests with this firmware, but andrew.w.farrar did some short tests without any problems.