I think the reason people here don't recommend the J-spec motor is because out of the box it has less potential than the U.S. motor. The U.S. motor comes with larger injectors and different cams (according to another post that I will put a link to) than the Japanese motor. We can BPU the US motor and not have to worry about fuel enrichment until we upgrade turbos. Also I believe the turbos on the J-Spec cars are smaller and the motor is overall tuned to produce torque at lower RPMs than the U.S. motor which has excellent top end charge.
Here is a post discussing some of the differences between the motors.