Currently restoring RM-914 VAR EU FR CV rom to my 520. Its about halfway done. Im using nokia care suite and the guide from XDA forums. Looking good so far.
EDIT: Nokia Care Suite said flash was successful but the phone won't boot. Just shows a blinking battery symbol. Will update when I know more.
EDIT 2: Ok. A few scary minutes later and a with few soft reset attempts (hold volume down and the Power button) the phone finally booted up and reached the setup screen. It was stuck at the nokia logo for a couple of minutes.
So yeah, its definitely possible to downgrade back to WP 8.0