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Here is another perspective on this topic from "Going Faster", the Skip Barber manual, which I was reading last night.
On p. 199, after a brief discussion on tread and how it relates to chunking and overheating in dry conditions, he states:
"Shaving the tire not only reduces the temperature of the tire and eliminates chunking, but it increases the CF of the tire. Showroom stock tires are typically run at depths between 5/32" and bald. In our testing we have found that CF differences from 5/32" down are inconsequential."
So, that is some reference to some actual data arrived via testing. Although the details of the test are not provided, I assume they took new showroom stock (unhardened) tires and shaved them to different tread depths and tested their coefficient of friction. No references to specific tires brands are made, but the term showroom stock clearly means street tires in our lingo.
A few paragraphs down, he also states:
"A tire's grip deteriorates with time and exposure to ultraviolet light."
Prior to that, on p. 197, in a section titled "Street vs Slick", he also states:
"For the most part, a tire's rubber compound determines its grip. Pure racing tires are generally much softer than street radials since they have an intended life, at most, of a few hundred miles. Street tires that last over 40000 miles have become common, and that longevity comes from a harder, more durable rubber compound. What you gain in longevity you lose in ultimate grip. A secondary advantage of a harder compound tire is that its traction doesn't vary as much over the course of its life as it would for a pure racing tire."
Therefore, an r-compound tire should exhibit a much more significant drop in traction with time and UV exposure than a harder street tire, and not vice versa as the "track nuts" Swamp spoke with at the track claimed. What Skip Barber says is much more consistent with my experiences with street vs r-compound tires.
In conclusion, Skip Barber seems to be saying that traction of a street tire has very little to do with tread depth as long as the depth is not more than 5/32", but more to do with the duration of use and exposure to UV. There are more nuanced issues such as slip angles, but those are not particularly relevant to this discussion.
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