The crosses I've made consistently show that lack of slow feathering results in arrow shaped lacing where two copies of slow feathering gives nice large oval lacing. I can easily take a few pictures of chicks with various combinations tomorrow if you would like to see the contrast.Good lacing really does not require Slow feathering(As in Barred Rock type of slow feathering).
I don't make any claim that the genetics I have are related to the variant of slow feathering in Barred Rocks. There are 4 documented slow feathering gene variants which can easily be looked up. There is even a wiki page on the subject. I believe the variant in my birds is k4.