350Z HR Y-pipe installed on a 2003 G35
#796
I've seen some pictures of the FX y-pipe and it does look just like the HR y-pipe. I'd say go for it.
#801
#803
As far as the Fx35 Y-Pipe I'm not sure if it gives the same power. I know that it looks nearly identical with the exception of the "hockey pucks".
#805
#809