It has nothing to do with the secondaries, well nothing really to do with them. The problem is at WOT or near that the engine opens everything up and for a short while some gas goes through the engine unburnt. When it hits the cats, the cats convert as much as is possable. For one reason or another the old cats seems to convert it badly. I don't understand exactly why this happenes this way, but this is what I have gathered.
