This is a ridiculous question. KG wasn't "added" to Allen and Pierce as far as the team identify goes...KG was the focus of the team's identity, and Pierce and Allen fit in around him. With Big Al instead of KG the entire team identity would have been different, the entire way that the team played would have been different, and the entire way that opposing teams approached them would have been different. Pierce's and Allen's strengths/weaknesses don't fit NEARLY as well around Jefferson as they do around KG.
And that's not even getting into the actual difference in talent level. Big Al's two greatest strengths are that he is an excellent scorer and rebounder...and KG scores more efficiently and can rebound better than him. Then you move to the other facets of the game...KG dwarfs him as both an individual and a team defender (and the entire Celtics D was built around this), KG is a much better team offensive player than Big Al, and KG creates more mismatches at both ends of the court than Big Al ever could. IMO, this isn't a realistic comparison.