Johnson destroys him. Kimbo couldn't fight worth a shit. Beating up bums with no real experience or training in street fights is a lot different than trying it with a professional. I used to LMAO when fight novices proposed that Kimbo could beat Tyson in a street fight. The Tyson of NOW would smoke Kimbo on his best day in any fighting setting - the ring or the street.
I think Johnson would get destroyed by most modern heavyweights but I overwhelmingly agree with your take in this match up, Slice was an atrocious boxer.
