Hi all,
I ended up bringing the car to the Chevy dealer to have it diagnosed. They first replaced the O2 sensor which I had replaced before. The car ran fine under light loads after that but misfired badly under higher loads. So sent it back and after more diagnosis they arrived at replacing the injector on cylinder 3 which had low flow. Made sense as cylinder 3 was the one mostly misfiring.
Car still had no power after that so they checked the cats and realized they had been burnt and clogged up as a result.
With the cats fixed the car runs like crazy again and the exhaust is as loud as it used to be.
So in the end the main issue was probably that the cats on my car could not handle the temps under track conditions because of my 760hp tune as well as the spark wires that did not have the boot protectors leading them to get melted which caused this chain of events.
Thank you all for trying to help me fix it! Sorry to report that in the meantime my employer decided to send me to Europe for 3 years starting this Fall. So now that my car is perfect again, I will have to sell it :(