It is true that one revolution equals one revolution no matter how far out the magnet is from the center of the prop shaft, but the dwell time will not be the same. This is because, as you said, the magnet will travel faster over the sensor pickup the farther out from the shaft centerline that it is. The chip inside the ignition module has some logic that says a dwell time (time it takes for the magnet to pass by the sensor) of 15 microseconds equals so many RPM. In your example, if your magnets were farther out from the center of the shaft, then for the same RPM, the travel time over the sensor would be smaller. Lets say 10 microseconds. Therefore the logic in the chip would incorrectly calculate the RPM. That is why I think that the sensor distance from the center line of the prop shaft is a factor.