An ECU tune can provide more power all along the rpm curve with improved throttle response. They can also stop the ECU adaptations that reduce power based on conservative driving habits.
How do ECU tuners do this? Factory cars have to have more conservative engine tuning so they can run on poor quality gas in some foreign countries. ECU tuners base their tunes on high quality premium fuel (typically 91 or 93 octane) and tightly adjust the engines performance parameters for maximum power and throttle response.
ECU tunes can sometime cause problems with emissions testing. For example, I had an alpha-N tune (a tune that does not use an MAF sensor) where the emissions tester could not read the ECU, and I needed a special waiver. Also, the dealer and/or car warranty may not be onboard with a non-factory engine tune. We recommend readily reversible tunes that can be loaded and unloaded through the OBD port to avoid these pitfalls.