When a friend of mine had greys fitted, they were overfuelling at idle. The car would idle fine, but blip the throttle and there was loads of dark coloured smoke. I know your engine is standard, but could your water temp sensor have failed, causing a rich mixture, which could also cause the high end misfire ? After sitting at idle for a few minutes, without blipping the throttle, what colour are your spark plugs ? If they're sooted up, would consider a new water temp sensor.