Cats are OK with grown chickens from my experience.  I just recently released 4 feral cats (mom & 3 kittens) that we had adopted.  They were penned up in an outdoor dog run for 2-3 months so they would re-home & the kittens could grow up, so they got to watch the chickens all day.  I watched them closely after we let the cats out.  Momma cat ignores the chickens but the kittens (~5 months old now, pretty big but still playful kittens) sometimes stalk the chickens.  The chickens definitely defend themselves very well, and we have two that usually turn the tables and attack and chase the cats away.  
I should probably mention that we did have one incident where one of our hens (our only banty) got a dislocated wing ~1 week after the cats had been released.  We don't know how it happened (nobody saw anything), but I suspect the cats might have had something to do with it.  We wrapped her wing for 2 weeks and she is fine now, but I still wonder how it happened.  I don't worry too much about them, especially since we have so many chickens.  If it were just a couple, or the cats outnumbered the birds, I might be more concerned.