There’s no way to tell how much better or worse he would’ve been than Coburn. But given how establishment choices drift well to the left, he might’ve been considerably worse. I’m going by memory on the polls, but I do remember Humphreys was faring so poorly that he scared the base straight.
http://www.ourcampaigns.com/RaceDetail.html?RaceID=21158
This page from OurCampaigns (which unfortunately does not have polls for a Carson-Humphreys matchup) has the discussion from the time, which reflects the concerns. It was curious that Humphreys carried a single county (where there were apparently only 39(!) voters) in the sw corner. He didn’t even carry OK County. I still stand on my opinion that Humphreys very well would’ve lost (narrowly). I do think part of the reason was that the rest of the state resented having a OKC candidate. KY has had a similar problem with Louisville candidates (although Mitch McConnell somewhat broke that model, but he was really Jefferson County).
Coburn sure did nuke his ass in the primary. I guess you could say if he got sh*t kicked that badly by Coburn in a race during which he was originally the front runner, that he may have faltered in the general enough to have lost.