EcuTek is not a piggyback system, neither is it a standalone ECU. There is no hardware required to be installed in your car.
EcuTek is basically software that can read and modify your standard ECU's parameters, such as boost profile, boost limit, rev limit, fuelling & ignition timing. The tuner will use a laptop to connect to your ECU through the OBDII plug, and then recalibrate these parameters to suit your car and its mods.
Dont look down on it because it is just software - the stock EVO ECU is already very advanced, and can be recalibrated to produce very substantial gains in hp and torque. Make sure that you do your engine mods first (intake, exhaust, turbo, injectors, etc.) before you do the EcuTek tune, so that you get the most out of it.
EcuTek supports EVO 5 onwards, as well as all turbocharged Subarus from year 2000 onwards.
One useful tip - whoever tunes your car, make sure you ask him to give you a dyno reading before and after the tune, so that you can have a comparison on paper.