If the gap needs to be narrowed when running the COP's with resistors, my guess would be that the resistance is to high that was added.
The current flow through the coil should be the same when either a COP or a stock coil is used. The reason for using resistors is the COP coils are about 35% lower ohmage than a stock coil. This allows circuit to conduct better, thus increasing current.
Current is normally flowing through the coils, A TCI interrupts the current flow when plug is fired. This is opposite what a CDI does. This makes it easy to measure current flow through the coils in a TCI circuit. Inserting an amp meter in series with stock coil will give current draw. Then insert a COP in same circuit and measure current flow again, it will be higher. Using Ohm's law, proper resistance value can be determined. I would assume it should be about 1 ohm.
Then the wattage rating for the resistor could be calculated by inserting the calculated resistor in line and measuring the voltage drop across the resistor. This test resistor could be too low a wattage for continuous use, but will survive long enough to get the needed voltage drop reading.
A better way to determine needed current flow for a COP would be to measure current flow through coil in a bike that was designed to use the COP coil, this would take out variables that are introduced when putting COP in a circuit not designed for them.
In the case of the Ignitech, once proper current flow in determined, the dwell setting can be used to adjust the current flow through COP.
I would think that if plugs need to be gapped narrower, that not enough current is flowing through coils, this would result in a magnetic field that is weaker when the TCI interrupts current flow. This would then create a smaller induced voltage in the coil secondary circuit thus the spark created would be less robust.
Attached are a couple of pictures when I tested coils using an Inductance meter. Static ohms reading for a coil are not a true reading.
Gary