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The authors of Going Faster most likely know more about driving on any kind of tire on a race track than you, I, or anybody else on this forum ever will. You asked for test results, I referenced their test result. Now, you're saying they don't know what they are talking about and their tests are not scientific. The test procedures are indeed not documented, but I'll buy their test results over any "opinion" expressed here any time.
Moreover, they bring up street tires because they seem to have racing experience with them!
"For the Spec Racer and Skip Barber Formula Dodge 2000 series, there are multiple reasons for preferring street tires to slicks. Safety and durability are high on the list..."
I don't think they run street tires in any of their current series though. However, their Formula 2000 racing school cars are on street tires. So, your dismissal of their expertise is perplexing.
And, if you really think the opinion of the average enthusiast serves as proof that decreased tread depth decreases traction, there are two people here who reported no "major" issues with grip on PS2s at the bars, and we've got the guy I know who turned faster laps at LRP on PS2s at the bars that were stashed away for several years than he did before when they had more tread despite not so significant loss in traction. As I said, the "significance" of the loss of traction is a subjective consideration, so opinions will not establish much either way. What we are discussing is if and how tread might be related to the loss in traction (as in the title of this thread "traction vs tread depth"), not if there will be a loss in traction (we've been over that and everyone agrees hardening will decrease traction).
Skip Barber manual says time is a factor, but it also says the effect is primarily experienced up front--that there is a noticeable drop in traction after initial use and the effect of time then somewhat stabilizes. So, they are not claiming a linear relationship there.
The UV exposure is more straight forward. Cars that experience tread wear must, by definition, experience exposure to the elements unless one wears the tread down on a dyno in an insulated garage. So, if UV exposure (and compound hardening) is the cause, the tread depth correlation can indeed be meaningless for the most part.
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