While I haven't used ferrite rings myself (but will shortly on my gasser) I helped a friend install some on his 86inch Canadair. There was definite jittering with the long servo leads but once the rings were installed the issue went away.

In his aircraft the engines were standard nitro motors so his exposure to interference would be expected to be much lower than a normal magneto ignitioned engine. I wouldn't expect the ferrite rings to block everything but they should help.

Luckily my RX is almost 24inches from the ignition source. Limited testing shows no intereference from a magneto ignition to my Hitec RX....however final testing is still required.

I wonder if a light alloy (printers plate) style firewall would help shield the interference to the RX? Just an idea but it should help minimise the EMI directed back towards the RX and antenna....