Bob - thanks for your comments. I've omitted some detail in the interests of brevity. I have only one mag, which has the AFS sensor already installed, and I have the associated wiring run through a "Dan Horton" style firewall penetration. My second ignition is a P-Mag with its tach signal not run through the firewall penetration.
My interest in continuing to use the AFS sensor is two-fold... Firstly, it works and works well, while I have helped several other owners/builders troubleshoot flaky indications on other tach sensing methods, including the GRT P-lead resistor pick-off. Secondly, by continuing to use it I can avoid significant rework to the firewall penetration. And then there's the third reason... I don't like the "blink" that happens to the RPM display as the EIS switches from one tach input to the other. With this in mind I think sticking with the AFS sensor isn't a terrible plan. I just need to figure out whether or not the EIS has the capability of handling the signal. GRT tech support has been unable to provide a meaningful response however I'll be contacting AFS tech support to get a better characterization of the output signal from their sensor before contacting GRT again.