I just got rid of my Nismo S-Tune...and yes..the aftermarket is better than the real Nismo exhaust. It does have better gains but nothing to brag about...which is why I recently got the F.I.
The actual exhaust coming off the Nismo won't sell for more than $800 TOPS. I just sold my S-tune for $500 with Stillen HFC's.
*edit* The S-Tune is better in sound...only has I think 7hp gain. I was only told is was better from the parts guys at Nissan..he explained but I didn't really listen becuz I originally purchased the ACTUAL Nismo but they intalled the wrong one