Replacing the stock ECU chip with a reprogrammed performance chip is a good option for anything up to a 10% increase in engine power. This is mainly due to the way in which the stock ECU chip is programmed. In essence, the stock ECU is programmed for optimal performance at peak torque so as to ensure that the car is drivable at low engine speeds. As most cars would be driven at low engine speeds of up to 3,500 RPM, ensuring that the car drives perfectly at these engine speeds is perfectly reasonable; unless you want to modify your car! A reprogrammed performance chip will be programmed for optimal performance up to the engine red line, releasing a moderate power increase but making the car less drivable at lower engine speeds.
This is all good and well if the performance chip is fitted on its own; however, fitting a reprogrammed chip to a modified engine for the purpose of accommodating the engine's new fuel requirements is a completely different story. The modifications carried out on your engine will alter the performance characteristics of the engine, as well as its fuel requirements and would require a chip programmed to meet those specific requirements. Thus, the chip will be specific to your car and could be quite expensive. Fortunately, some turbo and supercharger kit manufacturers will include bigger injectors and a reprogrammed chip under the assumption that you have not made, and will make, any other modifications to your engine as this will render the chip less effective.
Should you want to carry out additional modifications to your engine, you would need a different reprogrammed chip as you engine characteristics would change. Therefore, before you consider getting a reprogrammed performance chip for your ECU, you should first consider whether you will want to carry out more engine modifications at a later date. In such an event, it may be a better idea to fit a fully programmable aftermarket ECU that will allow you to reprogram the ECU as you make modification changes to the engine.