Cheika does like a line breaking winger, Speight is the best we have
Yeh, I've been thinking of him as a lock for a little while, but just recently I remembered how he wasn't picked so often last year and everyone was a bit surprised. Personally I think you need at least one specialist winger, and he seems to be the man for it. The only other really specialist options I can think of are Mitchell or Cummins (I still see JOC (James O'Connor)/Horne/AAC (Adam Ashley-Cooper) in more of the utility back cum winger role).
These are all reasons why I think Speight is one of the more certain RWC selections in the backline.