I had a discussion with Mark Shead once about this and im sure he said he did a test on his RS500 with a cone filter on it. It was 1 degree hotter inside the cone than outside. (if im wrong then Mark will say so). Make up your own minds as to weather a cone filter really does suck in that much hot air.
IMO 1 degree hotter is fuck all and will make very very little if any difference in terms of loss of power etc