Technician is right, installing a performance exhausts will not always result in more power - just a different exhaust tone. Some will throw around terms like reduced backpressure and lower restriction to exhaust flow. Sounds great - but the more important issue is proper exhaust scavenging, especially at low RPMs, where the 1.5L is already pretty taxed. What you will most likely see if a loss of low RPM power traded for high RPM power - this happens the majority of the time. Fuel economy loss is common - since the engine uses valve timing and exhaust pulses to enhance cylinder filling which will be broken when you change the exhaust system. This point - the ECM has to be reprogrammed to enable it to understand what is going on. The factory engine computer was only really designed to understand original components, adding aftermarket performance parts without changing the existing air/fuel/timing map usually ends up make the engine perform worst than before.