Fryer means a lot of things to different people. Before the genetics on meat chickens were figured out, fryers were the male leghorns which were hatched by the big egg producers. They would run them to 3 pounds live weight, then sell them as "young tender fryers" who were quite young. It was a way to utilize the males, rather than mechanically macerating them as they do now.
Since I'll be breeding my own layers now, too, the thought of marketing a fryer has its appeal. But, I will be doing many different breeds and cross breeds, so I can't give advice on anything in particular. Except, you could probably use any breed and get similar resulsts... but in the end, all purebreeds are going to seem a bit thin and weedy compared to the Cornish X.