In my experience, I'll get more rasp if the top reed extends a little more over the secondary reeds and if I reduce or eliminate the cut on the tips of the top reed. Don't extend too much. Also, red latex tends to provide more rasp over yellow (my typical ghost cut color). The spacing of the opening also plays a role, like you mentioned, but if the spacing is too narrow it can be hard to find the sweet spot for aligning the call in your mouth. Also, I'm not sure if this will work, but try moving the call forward in your mouth to provide less tongue coverage and more vibration (this works with purring). On a 2.5 reed ghost cut I usually get enough rasp but still hear the front end. It also allows me throw a jake yelp as well. Hope that helps.