its hard to tell really, as an N/A engine needs a certain amount of back pressure to work properly it might differ between the two.an exhaust is only as free flowing as the smallest part I suppose so it might not matter as they both go from 2.5" to 3". the sound might differ with different resonance and stuff. id go with the cobra personally as the quality is so much better than the japspeed. my japspeed looked like it had been run in the see after a week and blew at the flexis after a couple of months where my cobras been on for about 6 months now and looks nearly new