Callahan had a few good things going on offense, eventually, but it really wasn't that great or any better than what Pelini's teams have mustered, with or without Shawn Watson. Let's take a look and compare:
2003 (for reference): 24.77 points per game (7 games in which we scored 24 or less)
The Callahan era:
2004: 25.0 points per game (8 games in which we scored 24 or less)
2005: 24.6 ppg (5)
2006: 30.57 ppg (5)
2007: 33.42 ppg (4)
The Pelini era:
2008: 35.38 points per game (1 game in which we scored 24 or less)
2009: 25.14 ppg (7)
2010: 30.86 ppg (6)
2011: 29.15 ppg (6)
2012: 34.78 (2)
2013: 31.92 (6)
So... Pelini has already had multiple years with better offensive production than even Callahan's best offensive year (which also happened to be the worst season of our lives). And the 2009 offense, which was undoubtedly horrific, was better than two of Callahan's years. It took Callahan three years to even get better than the Solich offense that he replaced. Callahan had 5.5 games per season scoring 24 or less, while Pelini has had 4.66 such games per year.
I'm not too high on Pelini's coaching ability and am not too optimistic to get over the hump this year, but his offense, despite the number of mid-season changes or whatever, has not been any worse than Callahan's. Callahan's offensive prowess is a total myth.