It has been described in several different ways, but the most basic thing I can think of is this:

You will ONLY need upgraded injectors if the air-flow you currently have causes them to exceed their duty cycle. More air sends a signal to the ECU to ramp up fuel supply. The question is, how much is too much? Certainly supercharged applications require more fuel because they provide at least 50% more airflow through the MAF.

Rob from Needswings should be able to answer these questions to the precision you demand. Otherwise, we're all going to agree that we don't truly know how much of a waste of money they'll be on the N/A Crossfire, but we're fairly certain they'll be pointless enough that we wouldn't buy them. Yet.