Not sure what the 11" rotor kit is, Dr. Diff offers the stock rotors which are 10.95" or the larger later rotors with are 11.75". With 18's I'm guessing its probably the 11.75" kit. In either case, those are stock rotors so the offset doesn't change.
265/35/18 would be a little short for my liking. That's only a 25.3" tall tire. The 275/35/18's are as short as I like to go, and those are 25.6" tall. I almost went with a 255/40/18 because that's a 26" tall tire. But the 295/35/18's on the back of my car limited my tire choices a little, and the tire I wanted didn't come in a 255/40/18.