A certain importer south of LONDON, sells a lot of cars on part used tires. It is never advisable to drive a high power car on anything you know nothing about. Part worn tyres could have been really abused and could be years old you just don't know. I always wonder why someone would get rid of a tire with 6mm of tread all over. I got rid of my part worns after a week, saw cracks in the side wall not as bad as above. My tire supplier said the manufacture stopped supplying the Yoko's on my car about three years ago. The last time I saw cracks like those mentioned above was on a 5 year old spare on my caravan, it had never touched the road.
Get em off now, they are very dangerous, as Stu said it is your life, but might be some one else you take with you. If you get in a smack with that rubber the police will investigate and you could be on your own for mega damages at the very least.