I see what you are saying, but I think that even though the overall shape of the stock exhaust looks like an X, I would not call it an X-Pipe design and don't think it acts the same way.
True X-Pipes are basically 2 tubes with a small opening in the center connecting each other, right after this, they split into separate pipes.
On the stock design, gases come together on a single relatively long pipe and then it splits again. I think this would have an effect in sound and performance.
I guess what I wanna get at is that the hiss is caused by something else. Stock setup uses Y pipe and hiss is there... FI setup uses X pipe and hiss is there. Maybe its something with the engine itself, exhaust valves, gremlins. Lol I don't know.