I met an Evo owner last month, he has cat back and Ralliart ECU and I was bone stock at that time, both cars were running at stock boost but his felt a lot faster. So I decided to get some mod...
I recently installed a Tomei ECU and de-cat. It felt 10-15% stronger after installing the ECU, and another 10-15% from the de-cat. I was very happy with the result and so we met up again for another comparision. He immediatly felt the difference, he couldn't believe it's the same car! In a fix distance on one stretch of a road my car could easily reach 180+km/h, and we only got 162km/h on his car!
I am a bit worry about the hugh gain, I heard something about a/f ratio, I heard that it can gain power and hurt engine at the same time. Should I be worried?

 

i would get on a dyno and see what your A/F looks like. Even though they are the same cars, there are always going to be differences. You may have a strong motor, but your car may be reacting differently to the ECU. Take it from my own experiences with my old car, get on a dyno and find out what is going on for sure. A few dollars wasted to be sure is just insurance.