Gwern Branwen wrote: "Where does Kim Stanley Robinson say this?"
It comes from the blurb he provided for the back cover of the hardcover edition of Exodus from the Long Sun:
"Exodus from the Long Sun is a wonderful finish to a sequence that stands beside The Book of the New Sun as one of the greatest long science fiction novels ever written. Gene Wolfe is the King of the Slingshot Ending and this time the finale is a veritable Roman candle of heartstopping action and mind-boggling scale shifts. I read this book on a flight to Antarctica and kept forgetting to look out the window of the plane. Now all we can say is, Please start another one, Mr. Wolfe." -- Kim Stanley Robinson
He also talks about it in his Introduction to The Very Best of Gene Wolfe, a modified version of which can be read online at http://www.nyrsf.com/2013/09/a-story-kim-stanley-robinson.html