Well, its all about application. But, from a purely technical, all fighters being equal, point of view:
Kickboxing (American) outclasses Boxing for one reason: Body kicks. A front kick to the body/roundhouse kicks to the body control range and cause damage at a range a boxer cannot compete with. He'd be chewed up and probably KOed by a head kick.
Muay Thai Kickboxing/All non-American Kickboxing(s) vs Boxing/American Kickboxing. This one goes to Muay Thai and foreign (i.e. Dutch, French, etc) Kickboxing. In Boxing as well as American Kickboxing there are no leg kicks, so, the presence of leg kicks/knees and elbows gives the advantage here to Muay Thai and foreign kickboxing, but against any kickboxer the threat of the headkick is there. Against a boxer, again, he'd be chewed up and possibly KOed.
On the subject of Karate, Wing Chun, TKD, as well as any other less than sport oriented Martial Art, its not worth my time. Its not that I don't believe in them, I very mch do, I'm just so f*cking tired of arguing with ignorant haters about their application.
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. That’s it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit