We have only had Shields, Henderson, Overreem, and Diaz "come over". Shields before the buyout the other three after. Only Overreem and Diaz looked good but both got popped for illegal substances. So take that with a grain of salt.
Actually, Diaz got popped for weed, which I wouldn't consider a PED - so in his case, it's not like taking a sip from the Dead Sea