Put the car on "platforms" made of 6"x6" lumber and crawled under the car to see how much space there is between the tire and the coilover spring. On the passenger side I can easily slide the fattest hex bit through so let's say 10mm of space, on the affected driver side I need to squeeze the same bit through so let's say 9.5mm of space. I doubt the tire can stretch that much toward the spring but who knows? I got 25mm spacers just in case and will put them on to test.

Went to a tire shop and got two new tires installed, they looked at the destroyed sidewall and said because there is no evidence of the tire rubbing against the spring (no shiny circle like in the previous case) it is highly unlikely the tire rubbed against the spring. We both wondered if the sidewall cannot simply take the abuse with the added aero. One thought is to run more psi hot, I am usually at 28-29 hot so maybe stay at 32 or so and see if that makes a difference. Perhaps the sidewall compresses and uncompresses too much and gives out?

Pulled the front bumper to remove the splitter and installed M6 pre-bulbed plus nuts so in the future I can simply bolt and unbolt the splitter from under the car. Broke the install tool on the second plus nut and rigged my own install tool from a half-rusted bolt, a nut, and a washer. Lowered the car back to about 5" front, 5.25" rear. Going for another alignment Thursday, will run less camber in front and see if the driver front tire fares better this way.

Also, got enough supplies for the next 4 oil changes.

Dropped the camber in front and the driver front tire survived one day at Road Atlanta. :) Hopefully this fixes the problem, if not, will reduce the camber a tad more. Now onto building a custom splitter. :)

Lost 4th gear at Barber, upshifted 3-4 and heard a big crunch. Was able to drive back home to Atlanta but the transmission sounded like a concrete mixer the whole way. A replacement is here, just waiting for an aftermarket clutch to arrive.

The driver front inside sidewall survived with less camber, will need to add back .1-.2 camber though as tire temps were like 198-198-203 (inside-mid-outside) at 28 psi hot.

Ouch. How many days/events on it?

16 HPDE weekends, 9,300 miles, probably around 3,000 miles of which were on the track.

What is your transmission fluid maintenance schedule like?

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Damn. Are you planning on opening it up? Curious if the circlip went missing like the first gen boxes do.

Both transmission and rear diff fluid was changed last October at 3,6xx miles. I was planning to do two more HPDE's and change fluids again. I guess I will move to bi-yearly changes or maybe every other oil change (6 HPDE's). :D

May open up the transmission this winter if I am bored. I went through two trannies on gen 1, the first one lost detent balls, the second one was a used one that lasted 2 or so HPDE weekends so maybe it was already abused.

I went through a few transmissions on my 2014 BRZ. That tranny fluid was changed annually. I typically see about 20 track days annually.
Now that I have a 2023 BRZ I have been changing the tranny fluid after every track weekend. And I can feel a difference when I do it. I know it may sound excessive, but based on the experience I had with my 2014 BRZ and the CSG recommended maintenance schedule, I think it's the right thing to do.
I wonder if you would have experienced this failure if you had changed the fluid much more frequently.

New tranny is in, with some other upgrades:

• a slightly used South Bend stage 3 endurance clutch from @Irace86.2.0
• Verus clutch fork and pivot
• STi transmission mount (from my '17 BRZ)

OEM motor mounts are made of marshmallows; will install Cusco motor mounts that have been sitting on the shelf the last few months...

Cusco motor mounts are in. Also changed the rear diff fluid.

How do you find the new motor mounts ? I upgraded to the TRD ones but even those are a bit soft still

Only 5 miles on the motor mounts, we will see when they are broken in. So far, there is a vibration at around 1,500 rpms that you can feel through the chassis and the exhaust is noticeably louder inside the cabin. :D Otherwise, they feel pretty solid.