I think Kerry can be a very good actress, though she sometimes doesn't get completely lost in the character, but when she's giving it some I certainly couldn't complain about her acting, she's not as immersed as Idina, Julia or Cassidy I found, but she's still very good.
Ashleigh I've always found to be a little bland acting wise, I find her Elphaba a little generic. And about her voice, there's a quality to it that makes me think the songs don't sit very comfortably for her, sort of like a quiver, I can never describe what I think properly when it comes to voices, hence the best word I can come up with comes out and I just hope people maybe get what I mean lol. That makes me wary of thinking she could manage the role full time. I wouldn't have been confident that Shona or Cassidy could have managed taking over full time in the role either, though that's not intended as a slight to them as performers by any means!