an optical coupler only blocks the wire bound interference. It not necessarily blocks the airborne RF radiation onto the cable leads, that is why the 10" distance is advocated by all ignition manufacturers.
RTK did some nice tests, where the resistorless plugs radiated all over the place and caused problems, whilst the R-type plugs were quite safe in modern ignitions. If using the CM6 plug, which does not have a resistor, a resistor type HT cable could be used. I am not sure if this is the case with the ICU ignitions that MVVS now uses.