Well, when I heard Blackwater Park or Deliverance, I didn't think his growl could get better, but then he shows up on GR kicking fucking ass!
Also, not everything has to be a progression - I mean, look at Mikaels guitar playing. It hasn't increased dramatically (not that it's at all bad). Just like you don't expect him to pick up the violin in the name of progression, doesn't mean he has to growl less.
All of this is being said, while I haven't heard a note of Watershed, so I don't know whether I'll like the apparent "softness" of the album or not.