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Every Nissan that triggers a code 21 needs the Ignition Check feedback signal. Perhaps the MSD guy is talking about something else so make sure you both are "on the same page."
The capacitive discharge ignition (CDI), like MSD, uses a big capacitor, and some additional electronics, to store a very high voltage across the primary ignition coil. Discharging this high voltage will cause a larger voltage to be induced across the secondary side of the ignition coil. To get a similar effect from a pure magnetic coil you'd have to increase the coil turn ratios from say 10000:1 to 100000:1 or greater. Combining a bigger coil with a CDI will surely cause the regular spark plugs to wear out more prematurely. Unless you are boosting, have aggressive timing, or something similar like that, then most cylinders won't have a need for such an aggressive spark.
But going back to topic, pin 3 is surely going to have a larger voltage potential going into the ECU. Why? Because remember the primary side is used to pick up the ignition check signal and it will be at a much higher voltage potential. So that original resistor/condenser value will not work since you'd need a larger resistor to bring that signal back to the level that the ECU can safely sense. Do you know how the primary ignition signal is supposed to look? Check out picoscope. That signal is pretty much what needs to go to pin 3. You will need an oscilloscope to see the signal. Only then will you be able to tell if there are any large variations. If so, you will either have to live with code 21 or build a circuit to fix that issue. If that is the "extra part" that MSD provides then go ahead and get it to eliminate code 21. But make sure you know what that part is designed to do.
Good luck.
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