2,3 to 2,4 bar. I didn't experiment with higher but will try on coming track meets.
The course we were at on sunday is a very special high speed slalom that is very very telling when it comes to your car's balance and tire performance, here's a video from my PB (and some swedish text about TrofeoR
)
https://rejsa.nu/forum/viewtopic.php?p=1528494#1528494
EDIT: 2.3 to 2.4 bar equals aprox 33-35 PSI
EDIT2: your 38-40 PSI would then be aprox 2.6 to 2.8 bar. That is extremely high IMHO. Well, 2.6 is not but 2.8 is
