In general car manufacturers are smart blokes. So they have almost optimize the programing for fuel, timing and even the MIVEC. However withstanding this, I did some hardware mods, I cut off the cat., put in a free flowing muffler and the cheapest drop in after market air filter. I got 10HP tweaking with the original ECU. Now when you say you want to change your ECU, I gather you must be thinking of 2 things, either a reflash or a Piggy Back like Unichip. My last advice, unless you have hardware improvements besides what I listed like High Cams or a Bolt on Turbo, your gains will be restricted to about 10HP and in a heavy weight like a Lancer/Inspira, its not going to translate into good butt feel!