It's not the throttle servo extension that is sensitive to noise, it's the rx antenna. So keep that antenna far away from the ignition, any servo/power leads, or metal and it should be ok (not mentioning the metal to metal vibrations, that's another story). If the ignition switch is on it does not isolate the ignition battery from the ignition noise. The ignitions can send noise back to the ignition battery through the power wiring but todays ignition circuits are probably electronically filtered pretty well. The biggest source of noise on the ignition is the spark plug cable and even that is shielded pretty well. Watch out for bad ignition power connectors. They can vibrate with the engine running and that's bad news (intermittent continuity). I use the Deans Ultra for all rx and ignition power wiring on gassers. I tie a piece of nylon sewing thread around the connectors and put a tiny little drop of CA on the thread to hold the knot.

I'd keep the throttle servo on the opposite side of the engine box of ignition module. That way it won't be near the spark plug cable. And even so the throttle servo will still not pickup as much noise as the rx antenna. So I'd keep the rx back at least 12 inches. That's a good rule IMO.