There are still a lot of things I want to improve in mine, so I'm breeding back to black every other generation. Last year I produced only splits, because I didn't have a lavender male I was comfortable using. This year I will breed them to each other, keep only the lavender offspring, and breed those to black next year. I've seen a great deal of improvement in size, type, and feather quality but they still have a ways to go.
Another reason I like using a black or split male is gold leakage, which has been a problem in my line. Sometimes it's hard to see on lavender cockerels - my first year I had 2 that looked just a *little* brassy, so I used them anyway. By the time the were over a year old, they were almost 2-tone and, needless to say, I had brassy chicks. Leakage is usually obvious on a black male by the time he reaches breeding age.