I'll add some context since I watch a lot of Griffins.
Callahan is playing lower in the line up than guys like Ferraro or Sheahan. He usually plays with someone like Triston Grant. He doesnt get regular PP time (did get some when there were call ups) but does play the PK. Callahan is outscoring Sheahan and Ferraro while getting way less opportunity on the PP and with less skilled linemates. Hes essentially playing the exact role he would in the NHL on grand rapids, whereas Sheahan and Ferraro are playing more of a scorers role.
Callahan is arguably the most trusted winger down there defensively. Hes out at the end of games when the other team has the goalie out and everything. Overall I do like how Sheahan has played since his call up and he has looked great, but between Callahan and Ferraro I'm not sure who should really have the edge. Both are fast (Ferraro has elite speed) but Callahan adds more of that rugged play and seems to put the puck in more.
All three should be on the team next year. They all deserve a chance. Callahan looks like a better play than Tootoo down in GR. And if the wings want to be tougher you need to keep Callahan. Hes cheap, homegrown toughness that you dont need to overspend on in free agency for guys like Tootoo