I used an Ixworth over my Ross Cobb hens this year. Obviously the meat girls are not really great layers and I found that the quality of the eggs (strength of shell and shape) to be not so great, so finding GOOD eggs to hatch was tricky. from the ones that I have hatched out, they are down on weight and the meat on the breast is a lot less. I got an average of 7.5 lbw dressed weight compared to 10lbs on the straight Ross Cobbs at say 14 weeks, reared outdoors.
I'd be interested to see how yours come out as although mine were OK I'd really want more weight on them. I hope that at least that the offspring will produce decent eggs for further hatches and the hens themselves be more vigourous. The older Ross Cobb hens that we have kept on(my little lad loves them, they are so docile) are absolutely huge. I reckon they are at least a stone each. One has gone broody today too