You can already turn off most glass effects in 26 via "System Settings => Accessibility => Display => Reduce Transparency".

I had that turned off years ago (for reasons I don't remember), and was wondering what all the fuzz was about when 26 came out because I didn't see much of a difference ;)

IMHO the actual important visual changes in the 27 beta is that rolls back the bizarre oversized corner radius in Finder windows, and they also got rid of the 'every menu item must have an icon' idea.