It's not going to help him much. Modem images are about the same, it's just that China ROM is set to use the default SW MBN list, which actually allows for dual-SIM to work.
No, like I said above, it'll disable dual-SIM support.
You can try to change the SW MBN list to mbn_eea.txt, which will disable dual-SIM support, but will allow for VoLTE and VoWiFi to be auto-detected for your operator. Or, you can just set the specific SW MBN matching your operator for SIM 1, without changing the auto-detection list.
To access the "MODEM test tools", enter the secret code in the stock Dialer app: *#*#663368378#*#*, then "MBN config loading and activing tool", then "Advanced" button. You should see that "oem_sw.txt" is selected for the auto-detection list - set it to "mbn_eea.txt" and then "Validate" and "ok".
That should force the same modem behavior as on EEA ROM.
This process is true for all of Xiaomi's Snapdragon 865-powered devices.