I am wondering what afr's it generally takes to damage cats, Car i a 99 ls1 running on E10 93, with magnaflow spun cats.

 

It has more to do with peak exhaust temperature and exposure time to that temperature rather than simply hitting an A/F ratio and melting the cats.

As a point of reference a roadrace car may bake the cats after running several weekend track events, or a turbo car may bake the cat after one 1/4 run.

I think you would be hard pressed to do any damage to those cats at any A/F ratio if you are just using the car for occasional street fun.