You are going to buy the car or buy the 91 octane to run it on?
The point is that Honda aims the Civic at buyers who view a Honda recommendation to run on 87 as a good thing and view a Honda requirement to run 91 as a bad thing. So they tuned it for 87 and not for 91. If Honda wanted to max performance and tune it to require 91, they surely could have done so, and have done so on other cars they produce. They wanted the spec sheet for their economy car to say "Required fuel: Regular unleaded" not "Required fuel: Premium unleaded."
A 91 requirement on the Type R, or even the Si, and a corresponding performance increase, won't surprise me a bit. On the Type R I'd even expect it. So far in this thread my assumption has been we are talking about the Sedan, Coupe, or Hatchback variants available today.
If you want to run it on 91, knock yourself out but just don't expect to be able to detect any difference except in your wallet. I'm sure the gas companies will be glad to sell it to you.