It does seem that until younger Willis begins to blossom or Vailanu hits his stride, we're short on 8's.
Shields has the bulk, Willis the explosive power off of the base and Youngs the hands of a good 8. Ideally you'd want all 3 on the pitch, but just feels a bit out of balance. That being said, you could have them vary who packs down at 8 based on attacking/defensive I guess