OK. Am I missing something here?
OP says:
It only leaks about 1/2 quart at the right rear wheel after hard high RPM autocross runs.
I replaced the coolant cap with the newer version, but no change in problem.
I pulled out rear carpet and checked tank for any leaks with engine at maximum operating temperature with the fans running, but no leak there.
I'm only finding coolant on the ground after autocross runs. Driving around on the street will not cause any leakage.
That wouldn't give me the impression of any spider leaks or whatever.
I'm going to guess that OP drives his car hard... perhaps there's an air bubble... needs to properly be bled out... and back to normal.
Does not the coolant leaking at the passenger rear wheel provide at all a clue? Doesn't a leak follow the path of least reistance?
I'm not trying to be schmuky, it comes natural.
I drove my Boxster home from Vegas, a couple weeks after purchasing it in 2017, like a ****************stain on a new pair of undewear and when I, literally, hit my driveway it dumped like a transient on a San Francisco boardwalk.
A new coolant cap... some distilled water... and letting the relief valve do it's job for a few days.
However, OP should follow Rob's adviice... check under that piece of wood on the trunk floor board... although my guess is that when the OP "pulled" the carpet the wood was evidentr and begged to be lifted up, checking underneath.
Good day, gentlemen.