Paul/Johnson would, jokes aside, very possibly get 2-3% of the vote, which is relatively impressive for a third party candidate.
Paul could get up to 15-20% if he goes independent. If he actually takes the Libertarian nomination, it might actually hurt him. Remeber, the purist like Mary Ruwart want legalized assisted suicide, and open borders. The party holding these kind of views can only hurt Paul. If anything, Paul takes the Constitution Party ballot line.
You think that a candidate who can't manage 8% in the GOP primary is going to get 20% of the vote as a third-party candidate???