I agree that he needs to beat a top ten MMA guy before you start ranking him top five or even closer
Biggy - The problem with Overeem isnt his skills or potential, but its his mental fortitude. When his opponents start weathering his initial storm he quickly begins to fade. Adding on extra muscle didnt help for Vitor at any of the heavier weight classes, nor will it help for Overeem when/if he fights a legit top 5 fighter.
This is just a nice way of saying he has no heart which is why he has a dozen losses in MMA. No amount of steroids or K-1 is going to give him a heart in MMA. He's the games biggest front runner and he'll only be top 10 if he starts fighting legit comp instead of crushing cans.