Johnson isn't the same character he was in the London Mayoral race these days though. A lot of people have already formed an opinion of him and he has become a bogeyman of sorts for Remain.
I think he would win a majority at the next election though but because of how unpopular Corbyn is. Even with the Tories in an utter state they're still largely matching Labour so any sense of direction and structure from the party will probably bounce them over the top. Plus the voters who left them will likely come back if Corbyn looks to stand a chance of getting into No 10.
What's interesting is his seat, Uxbridge. If (let's face it when) he comes PM then Labour might well focus on it. He has a decent majority at 10% but if there are many Tory > Lib Dem defections then it becomes a contested seat. London +
increase in young voters there + Brexit might make it a contest albeit a long shot.