I would disagree and see a definite correlation between not just franchise-level QBs and their Wonderlic scores but "winning" QBs in general
First to your point re UFA QBs.
One site lists the following 9 (actually 10 but Warren Moon was discriminated against so he doesn't count) I could not find corresponding wonderlics for Dave Krieg, Jon Kitna, Jim Hart, Mike Tomczak, Jeff Garcia or Eric Kramer but I did find them for Kurt Warner(29), Tony Romoa(37) and Jake Delhomme(32). I also determined that the avg QB score is 24 http://www.realclearsports.com/charts/starting_qb_wonderlic_scores-20.html
So I would say that in general, intelligence helped some of the more successful UFA QBs overcome perceived weaknesses. OTOH, the ones no one has heard of were considered enough of a prospect to be administered the test and perhaps the test only confirmed that in a position even in college where smarts help, most of them fell short.
Now as to Jeff George with the Golden Arm - wasn't it his intelligence or lack there of that was his undoing? Ditto Vince Young whose impressive athleticism could not overcome his mental inabilities. In hindsight, knowing what you know, would you have spent even a 3rd rounder on either of them?
And that's the point: there is not a single QB of the 16 who scored 13 or below registered prior to Jackson's 13 who knowing how it turned out would not justify even a 3rd round pick.
Dallas in the 1970s was the first team to administer the test so conservatively speaking, it's been used by the NFL at least 38-40 years. In that time if a conservative average of 4 QBs per draft were taken, that would be about 160 QBs. If 16 scored 13 or below, that would be 10%. Really, if you had several $millions to wager, barely the equivalent of a 1st or 2nd round picks would you take a 1 in 10 bet that any scoring 13 or below would be successful?
And that would be zero out of est. 160 who turned out to be a franchise-level QB.. Would you wager that amount that after all this time and QBs, this would be the first anomaly? I think not. IMO you actually stand a still rather statistically irrelevant but better chance of getting a franchise QB by signing a high wonderlic-scoring UFA.
I agree intelligence helped some of the more successful udfa but at the same time we can't determine if a lack of intelligence was the determining factor in the lack of success for the one's that didn't make it. As an udfa it could be simply they didn't have the physical tools required as in not having an NFL caliber arm or other physical traits that are desired. It doesn't matter how much intelligence or heart you have if you simply don't have the arm to make NFL caliber throws.
I think it's fair to say if a college QB has a 1st round grade based on game tape and great combine numbers; teams would disregard a low wonderlic and grade him as a 1st round pick.
Also intelligence can mean different things to different people especially when it comes to sports or non traditional academia. LeBron James has one of the highest B-ball IQ's ever but if you ever heard him speak he sounds like someone who got by in HS based on his athletic gifts. So the question is can someone have a high football IQ but a below average IQ outside of their sport. I think the answer is yes. Now correct me if I'm wrong but I don't believe the wonderlic has anything to do with football related questions.
I used to give people an aptitude test for a living and while some people failed due to just being dumb other's failed for a lack of preparation or just not being good test takers or other reasons. I think the same can apply to some low scoring wonderlic testers.
As far as I remember Jeff George was a selfish player who lacked leadership.