yup, you are absolutly correct. I have a 2007 G35 Coupe 6MT, with Rear Active Steering. I have done some research as well, and even contacted HKS. We can not use any exhaust system that deviates from the stock exhaust pass.
I have just ordered tanabe madalion touring with y-pipe today. I took a gamble on it, because no one knows if it is compatible, due to the fact that RAS is so new on our vehicle. It comes in next tuesday, I'll keep you posted!