Hi, I have done exactly what you're trying to do and it all worked a treat. I unlocked the bootloader via the offical Sony route then flashed the leaked ROM. Then after 3 weeks I have flashed the handset back to world generic firmware and relocked the bootloader. It then used offical Sony feeds to upgrade itself back to the latest firmware (It won't do this unless the bootloader is relocked).
The leaked firmware is very good and gives the phone a new lease of life, but I had an issue with the proximity sensor not turning the screen off during calls (which is fine on ICS). now waiting for the offical release.
