In the words of Leaf GM Brian Burke, "We expect he will see significant time in the top six." The question posed, is MacArthur a legitimate top-six NHL forward or only on a team as challenged at the forward positions as the Leafs?
MacArthur is predominantly a left winger, but can play centre as well. Before his signing, the Leafs top two LW's are Kris Versteeg and Nikolai Kulemin with their top centres being Tyler Bozak and Mikhail Grabovski. Of these four I can easily see MacArthur supplanting Kulemin or Grabovski on a second line perhaps only for the fact that he's a good old Alberta boy slightly more in the mold of a Burke/Wilson type of player. In the event that Tyler Bozak fails to carry the mail as first line centre to Phil Kessel, I could even see MacArthur getting chance on the top line.
In his three junior seasons with Medecine Hat, Macarthur notched 75,75 and 74 points and then proceeded to average a point per game over two and a half AHL seasons. Obviously, there is some scoring talent in his overall make-up. His NHL scoring rate is about as consistent as you can find. Over portions of four big league seasons his points per game rates read; 0.37, 0.41, 0.44 and last year's 0.43. Prior to his trade to Atlanta he scored 0.43 Pts/GP with Buffalo and with the Thrasher, yep, 0.43.
At 25 years old with a prime opportunity on the horizon, MacArthur is quite capable of increasing his scoring rate with additional ice-time. This is most likely an opportunity he would not receive with at least two-thirds of the teams in the league that can put forth six forwards of higher calibre than Clarke MacArthur.
My prediction, he plays as much centre as left wing and produces somewhere in the neighbourhood of 18 to 20 goals and 45 to 50 points. Sadly, these are indeed top-six numbers on the Maple Leafs.