[toyos]

I have this image by Mark D. Russell, Ph.D published on Sky & Telescope years ago about the effects of central obstruction. There's an obvious difference in image quality between a scope with 0% obstruction to one with 18% obstruction. I saw similar results based on my own objective observations with various telescopes since I often bought telescopes just for the sake of comparing them myself (sold them again at a loss soon after). In my opinion, central obstruction is like your vehicle's aerodynamics, the lower the better (in most cases); there's no definite cutoff to it.