rpike,
I've been flying ignition powered engines in giant scale planes since 1982 and have always kept my receiver system (receiver, switches, servos, batteries) as far behind the firewall as possible and have not had interference problems from the ignition. As of late I've noticed that there is a trend toward mounting the throttle servo in the engine compartment especially where people are using ZDZ rear carb engines. This is probably so that the most precise control can be obtained for the throttle position. Some are using opto isolators to isolate from ignition noise. I'm not really sure how well this works.

I have a ZDZ80 on order and plan to make a zero slop 90 degree bell crank to use with it on the throttle unless I get my mind changed. That way I can mount the throttle servo back in the cabin area in the usual place and continue to maintain the RF isolation.

Sure hope Ralph (Rcign) jumps in here with his words of wisdom.

Ken