I had Magnecor when I got the car and they were terrible, maybe they were just old, but I was advised by a tuner that they were rubbish and he had nothing but trouble with them, I now have genuine Ford leads (Alot more expensive though) and not only do they look better imo they also work better, so thats my experience, as said though re interferance so long as they are supressed you shouldn't have EMI issues.

Martin