Remapping ECU

[bwdz]

Easy to do on Turbo engines as you are upping the boost supplying much more air to the engine. Your engine's air supply is fixed unless you make some hardware changes. You can optimize fuel delivery and timing because manufacturers have to be somewhat conservative with it because it has to run right in all weather conditions at all elevations etc.. and it has to be smooth and reliable and compensate for you occasionally getting poor quality fuel and so on and so forth.
You can gain a few hp from remapping with more aggressive timing curves but it won't be all that much, probably 10-15hp peak with maybe a nicer midrange torque curve. Oh and be careful of tuner's ads for hp gained with their tunes. Back in the 90s I worked in a performance shop and ran a flowbench and dyno and we made some aftermarket parts for Mustang 5.0s. A lot of the chips we tested that gained 25hp advertised only did so at unusable rpm long after you shifted, peak hp would be like 3 or 4hp gain. Some of them would get down right scary lean conditions.