No, we don't know his impact in the locker room, but that doesn't mean you can dismiss it either. When the team struggles, fans pound the "lack of leadership" drum until their hands bleed, but when it comes to trade value, it's virtually meaningless? Not buying it.
The team also faced a significant number of teams post trade deadline who were well on their way to missing the playoffs. Of the 20 games the Rangers played, the 13 of them were against teams who missed the cut. A few of them finishing the season well below .500. So, once again, "the numbers" -- and "reality" apparently -- can be spun to suit just about whatever argument you want to make.
No, Callahan has never been a prolific playoff performer. At least not on the scoresheet. But he was a more potent performer in the 20 regular season games with Tampa than MSL was in his 20 games with New York. So how can one say with any certainty that Callahan would not have had a similar impact on the offensive success of the Rangers during that same span? Again, it's all subjective.
The argument could never, and would never, stay within the context you specified. Trades aren't examined in that sort of vacuum, nor should they be. Whether people want to admit it or not, the success of that trade is going to be determined by the success of the Rangers. The deal was made to increase their odds of winning a cup. So that's the measure for me. That's my position, but as per usual, I'm certain that will be construed by many as me hoping for the team to fail.