When one of my cornish x's wandered up the hill and towards the house this summer (she was about 4 lbs), my cat Bandit (my usually mild mannered, 8 lb kitty) attacked her. Luckily I had seen her low crawling, stalking her, so I FLEW oh my goodness and made to the chicken within a second of Bandit, but Bandit literally bowled the hen over she pounced so hard, and had I not been there, would have most certainly hurt her. I was shocked as Bandit had never "appeared" to pay attention to the larger broilers (little chicks of course, but that's expected)...she used to watch the full grown layers several years ago, but lost interest quickly and hasn't even glanced at them since... yet 2 months ago attacked my broiler?!?! So yes, in short, you do have to worry about cats.
-Jessa