I've always adjusted the performance slider downward to match my actual chrono speed. And it makes sense that On Target would recommend a lesser spined arrow for an "under-performing" bow.

I'm not sure I understand why somebody would want to use your method. Whether you decrease IBO speed or slide the performance slider down, the concept is the same: Your arrow is leaving the string at a slower speed. The dynamic forces acting on the arrow that cause it to flex have been decreased. So your recommended arrow spine SHOULD change. Why would you want it to NOT change?

Unless I'm missing something, this makes no sense to me.