I think this upcoming album will definitely sound better than DM, but that's not saying much at all
IDK, for me, the best producer from strictly a sonic perspective for Metallica is and will always be Bobby. No one else has ever come close to making Metallica sound as good on record as him. I know Greg makes "suggestions" here and there, but I'd be curious how much influence he had during this process on things like arrangement, tempo, parts, melodies, etc... If his influence was minimal, then his main function is essentially related to the sonics of the album. If that's the case why not bring Rock in?