I think the signal from the ECU is possibly been pulled down too much from the extra coils. If you can get your hands on an oscilloscope you will see if that is the problem.
If there is a 5-volt signal coming out of the ECU to the igniter, you can easily use an Arduino to boost the signal, just give every coil its own output pin on the Arduino.
If you read the fine print in the link above, it said that they used the original igniter and coil so the tach would work, maybe that would be a good place to start, disconnect the original igniter, and see if that helps by reducing the load on the ECU signal. If you need the tach the Arduino could supply the signal for it. (I've used Arduino to drive a speedo before a tach is basically the same thing)
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