Here we go again. Bottom line is...... If you dont wanna spend alot of money on AEM, do it. The only argument some people make is reliability. I personally know people who are tuned my Martin and have had the car 5+ years with no issues at all. There is more piece of mind with AEM. To do it the RIGHT way go AEM but your looking at over 1k, which is why most go ROM tune. Either way yes you will see sufficient gains when you have the proper tune for your set up. Not sure if an s15 cost any more than an s13/4. That is probably the only unbiased post youll get on this topic lol