Be aware that the 12V side of the coil is also releasing an inductive spike into your circuit. Not as substantial as the spark plug side but still in the 100s of volts. Your FET should be one designed specifically for these types of ignition circuits.
On the hall sensor make sure you have a ~100 ohm resistor on the V+ line and a small cap between the V+ and GND (these recommendations are typically described in the datasheet for the sensor). It wouldn't hurt to add a ~7V zener from GND to V+ as well.
If it is RFI make sure you're using a resistor spark plug. Plugs manufactured without a resistor shouldn't even exist but there's still a lot of them out there.