He is running at Bradenton track where Tesla Plaid Channel runs his car at, and TPC posts videos and timeslips all year long from there, while having more dragstrip runs than any other plaid on the planet. You can watch TPC's timeslips change a little but throughout the year when the track conditions and climate changes.
If you take TPC's timeslips and compare them to Brooks' timeslips, you can see the 60' times are pretty similar, and then the differences start occuring past the 330' mark, where a wider less efficient tire profile would hinder the plaid. The second thing is as you use tires, the silica is worked out of them, and a wide, low profile tire would be more susceptible to the detrimental effects of hard capping (wheels spinning on the top end under full load where the track is less sticky) if they have been run a lot. Whereas tires with larger sidewalls could compensate for that with their pliability.
But all-in-all, Brooks' times are pretty close to what the car should do with OEM 21" tires, they always trap a little slower than the 19" on a decent surface. I see 0.2 slower at the Vegas dragstrip on the 21" versus the 19" on average with my car.
I really don't think he is seeing any wear, or anything beyond something that can easily be attributed to track conditions or his tire setup.