There's been quite a few world champion boxers with kickboxing backgrounds.

Vitali Klitschko was a 6 time (amateur & pro) world kickboxing champ.

Alexander Povetkin won a world Junior Amateur championship & a pro European kickboxing title before transitioning to boxing.

James Warring - Former IBF Cruiserweight champion was a four time professional kickboxing champ. Defeated Vitali in an amateur kickboxing bout.

Marco Huck was a two time amateur kickboxing champion.

Troy Dorsey won the IBF Featherweight title in 91. Had some close fights with Jorge Paez back in the day. The only man (according to wiki) to win world titles in Karate & Boxing. His kickboxing credentials are too long to type so I'll just post his wiki page Troy Dorsey - Wikipedia, the free encyclopedia

Other guys like Jerome Le Banner, Rick Roufus, and the late Mike Bernardo boxed. They had moderate success...with Roufus & Bernardo winning minor titles.

Mike Bernardo - Boxer
Rick Roufus - Boxer
Jerome Le Banner - Boxer