To a large degree, it comes down to racquet acceleration more than anything else. Full poly and or gut/poly may enhance spin, but if the feeling on impact causes you discomfort or worse, pain, you're going to decelerate through the stroke and actually lose spin.
Now this is worsened by the fact that polys play great to begin with, but some time, you notice your arm just isn't as fresh or worse, hurts. When this happens, you naturally avoid what hurts and consequently your swing speed slows down.
I found that out after going on a lark by trying a full bed of syngut. It didn't last too long, but right off the bat, I noticed that it felt like butter, cupped the ball, and I could swing through without hesitation.
Why didn't I convert? Mostly performance. Control wasn't there and I had to really swing all the time to get it.
There is definitely a sweet spot in your string set up. My advice is not to rule anything out.