I wouldn't say I tried everything possible but in the end I choose another performance files-set: I figure the problem was that the cars speed didn't fit to the track... they simply were too slow (I had pimped the *.gp3 with GP4Phys...).
I had laptimes of 1:37.0 for the Zhuhai track - now the best car (not mine unfortunately...
) rounds the circuit in 1:20.0! From initially 21 cars now approximately 2/3 survive although some encounter several excursions to the gravel which often causes unplanned pit stops... I would say: problem solved!
Thanx for the engagement Turbo Lover and salvasirignano!