Good question, and I looking forward to seeing the answer.
In the meantime, remember that transfers can impact programs in a number of ways: (1) obviously, some transfers are better than others; I would argue that picking up a one or two 3-star players has more significance than picking up five no-star transfers, even if all five were to become starters; (2) one or two quality transfers, who do become starters, often times allows a coach to move the 'original starter' to a position that is better suited for his skill/talent level; and (3) along those same lines, a couple of good transfers and the inevitable shifting of players could knock a team's glaring weakness right to the bench. Also, keep in mind the positions that the transfers play. If the player transferring in is a quality kicker, for example, and his new team doesn't have a returning or rising place kicker, the transfer could make a 5-point per game difference on average. Finally, transfers at the high school level can add much-needed depth to a team, perhaps giving key personnel critical breaks in big games.
Oh (in the spirit of Steve Jobs), one last thing: just because the transfer isn't named a starter over the son of Booster Joe really doesn't mean a whole lot if the transfer ends up playing most of the game and all of the critical downs.