I bought a setback post with the idea of stretching out my saddle to bar reach on a too-small frame that I just bought. I tried for 6-months to like it, but I finally came to the conclusion that KOPS is a completely independent adjustment that you need to figure out for yourself. For me, I need a zero setback for things to feel right. Opposite issue from you, but if your knees feel ok right now, I would suggest changing stems first.