So, it looks like the firmware I posted is the same as the one on the Yi website (CRC32 match).
And the restore (emergency) firmware and normal firmware are a CRC32 match. Depending on how the firmware is named must do different things during a firmware update (as someone used this firmware and ended up stuck with japanese language somehow). I'm too chicken to try installing the 'restore' firmware as it must overwrite the bootloader and maybe wipeout the english settings.
Either that or there is a an option in the chinese version of the app to enable h.265 which is hidden in the english version.
UPDATE: I de-compiled the Yi app and found a check in the setting screen to only show the H265 setting when CN was in the serial number. I changed this check to EU and was able to enable H.265 encoding.
So far so good, filesize went from 160mb to 60mb per minute of video, it was using the H.265 codec however the video unusable. It keep blanking out every 5 seconds or so going to white and freezing. It was still using H.264 encoding for the smaller 'preview' video.
Maybe there is a difference in the hardware after all?
Here is the modified version of the android app if anyone else wants to try it.
Yi App EU H.265
You need to uninstall the current version first and allow unknown sources.
For reference the file I modified was com\xiaoyi\car\camera\mvp\presenter\CameraSettingPresenter.java
Line 100
if (!TextUtils.isEmpty(this.cameraDevice.realmGet$deviceSn()) && this.cameraDevice.realmGet$deviceSn().length() > 8 && this.cameraDevice.realmGet$deviceSn().substring(6, 8).equalsIgnoreCase("
CN"))
... to "EU"