Fujiyama (kyokushin) vs. Botha (boxing) in K-1.
Fujiyama is a K-1 regular and dropped Botha with a kick to the head.
Botha (boxing) vs. Le Banner (kickboxing)
Botha, learning from his experience was able to keep Le Banner in a tight boxing match and rock him silly.
Ray Mercer (boxing) vs. Le Banner (i think??)(kickboxing)
Le Banner drops mercer with a head kick.
Botha vs. Abidi (kickboxing)
Part 1 - Botha clearly dominates, blasts Abidi and sends him to the mat, but gets dq'd for hitting abidi on the ground. Botha would have won by KO or TKO in rd 1 if they discounted the ground hook.
Part 2 - Abidi does his homework and puts up better defense against Botha, peppers him with some slamming thigh kicks and sends him to the mat with a kick to the head.
There's more...
but its pretty even. Most of the boxing vs. kickboxing/muaythai/kyokushin fights are usually between former pro-boxers and K-1's top.
Will we ever see a fight between a current boxing champ vs a rajadamnern champ? Probably not because fights are as much about economics as they are about proving ground...