About the same... a little sloppy. For 17x9 I would not push beyond 255 wide.
Below is a picture of a 235/40/17 R888 on a 17x9 RPF1. Notice nicely supported vertical sidewalls. When sidewalls are straight up and down you get fast response, stable contact patch, more grip from the tire, etc. The car is easier to drive at the limit when the tires are not sloshing all over the rim. For autocross we often shove more tire on the rim than is ideal because we have a limit on rim width for many classes. We compensate for the mess this makes with higher tire pressures. The result MAY be faster around the autocross course...but it can feel numb, sloppy, and if not for the rules on wheel width we would not be doing it!
Thus, you could do the 265 on the 17x9 if you are running CSP but I suggest no more than 255 for that rim. And if you really want to lay down 265 size tires, get a 10 inch rim. For example, our NC Light track project MX5 started with 9.5 inch wide wheels....with just a 255 wide tire. Recently picked up some 17x10 for NC LIGHT....to run 265 tires.