I cannot tell if anyone responded to this question. Here is a link that was helpful to me. It is actually about marans I believe but the slow feather gene info must be pretty much the same for any chickens.
http://www.marans.eu/genetiqa.htm
Scroll down a little to "growth or speed of fledgling".
Thank you for replying and for the link! I will tackle it in the morning - with a LARGE cup of coffee!