Pre USADA Johnny was a huge WW whose walk around weight was 220. Yes I expect that Johnny to beat Khabib just based off TD defense and beating him on the feet especially in a 3 round fight.
But I will say this, Khabib can engage a grappling heavy plan, wear Johnny down in a 5 round fight and proceed to sub a gassed Johnny in the championship rounds. Like Rumble vs DC 2. Rumble won the first round via wrestling against DC but in the process basically burnt his tank completely out and had nothing for DC in the second.
I wouldn't be surprised if either guy won honestly. Post USADA Johnny, even if he's 185, is getting waxed by Khabib.