It's not a question of tolerance.

They are two different devices. The IBEC (Ignition Battery Eliminator Circuit) is what the name implies. It has optical coupling and ignition kill capability. My TechAero IBEC has a built in regulator to reduce Lithium battery voltages to 5.0 volts. It is rock solid. There is no question of RFI. The opti kill has one function only - remote shutoff of the ignition.

So the answer to your question should be obvious. If you are going to continue to run a separate ignition battery, forget the IBEC and get a smart fly opti kill.