I think it will be close. Herbert is burning through cash at a rate that shows more concern than the polling would justify, but all of the polling that has been released publicly since the convention is polling Herbert has paid for and they refuse to release their protocols, sampling methods, or questions--they just have the pollster (Dan Jones) do a one paragraph release saying "we polled some people, and Herbert's knocking them dead!" Meanwhile Herbert pleads and begs for more money so he can fight off the guy he's decimating?
It was the same story before the convention--Herbert's Dan Jones poll said he would win and avoid a primary, but Herbert was out there putting on concerts at Thanksgiving Point, renting out the RSL Stadium, and just spending oodles of money....not the behavior you'd associate with a guy who believes those poll numbers. And then he was 10+ pts behind at the convention.
I think it is much close than these campaign polls would have you believe, and I think Johnson is running a very smart campaign with how he is nicking and poking at Herbert and letting Herbert hurt himself (i.e. releasing the damning recording of Herbert telling lobbyists he doesn't agree with their clients' positions, but if they pay over $1000 to his campaign, he can guarantee results they will be happy with, and letting Herbert twist in the wind to try and explain it away). Meanwhile he's keeping his powder dry for a late push in mass marketing so he hits maximum exposure on primary day.
I don't know that most GOPers prefer Johnson, but he just needs to get Herbert voters demotivated and rile up enough others to get to 50.00001% of that small subset of Utah GOPer's who actually vote on primary day, and I think he's got a very smart plan to get there. I hope he does.