Sorry to revive this thread, but I have good news!
I took the dive and updated my bios to 5603 from 4207 and have my passthrough working just fine!!!! I was actually shocked.
I believe it is because they updated to AGESA to 1006 in bios version Version 5601 which is the same AGESA as in bios 4207. ( at least that was my thought prosses)
Glad they finally released a working bios for passthrough after many years!!!!