95% of RFI comes from the Spark plug. Shielding and resistance will greatly help.

This next bit will probably start a food fight but here goes. You should be able to fly an gas engined plane on FM. If you have a "RFI Problem," and you fix it by using PCM. You really didn't fix it, you hid it and sooner or later it will bite you in the b*t.
If your flying FM and want to change to PCM for some of the other functions thats cool.