We plant rye on 15" and still get good results. The only reason they are on 15" is because that is what we have available equipment wise to do the job. I used to double plant when we were doing covers on a smaller scale but now I don't have time to run across it twice. My other thought is on 15"s is I can come in and strip between the rows of rye and not destroy as much. Ideally I'd like to go to twin row rye eventually.