I'm trying to downgrade it to 4.1.1 so I can try this unlocking method but getting stuck with the Samsung logo (a boot loop??) when I try to flash any stock 4.1.1 ROM (have tried 3 different ones so far - EE in the UK, Vodafone NZ and Portugal)
Have tried via Mobile Odin, desktop Odin and CWM Recovery with no luck.
With CWM an actual error message can be see which is the "e:signature not verified" message. But either my googlefu is weak or there simply isn't that much info on this error.