Cool head: runs a tad cooler, allows for proper domes that burn more efficiently and at your desired compression and are swappable for different set ups. Where as the stock head needs to be machined and cant keep being re configured.
Radiator: sheer physics, bigger radiators cool more. Simple.
Billit impeller: almost never fail while stock impellers are very known for having issues like melting, snapping at the shaft, or losing fins. To boot the move more coolant and really do drop engine temps.
All in all, who ever told you that shit is quite wrong. Just sounds like some one who has had semi stock banshees on and off for years and has never really looked at the aftermarket other then what their local shop sells at the counter. I've met so many guys with banshees who swear they have the baddest bike around till we ride together and they see this website. Then I have to deal with the constant regurgitated misinformation that they find on here.