I am now curious as to what my GTI's top speed is now. I would like to find out but I would probably need a runway to do it, or some vacant highway out in New Mexico or Nevada or something. The 120 mph comment is amusing. Mine hits 120 so damn quick its almost comical, and then I still have 5th and 6th gear lol.
Back to the tune/ecu thing - since I am an I.T. guy I will put it in that perspective. Say you want to protect your computer right? You install security software that "talks" to the operating system, allowing you to add email scanning, website monitoring, turning virus detection features on/off etc., that is VCDS so to speak. Now say you want hard drive encryption, this is where you go into the BIOS firmware of the motherboard (think ECU in car terms) and enable the TPM module so that Bitlocker can work, or you enable a BIOS password that locks the operating system out upon boot, until you enter the password. Also firmware update to the system board in your computer would be comparable to an ecu-level tune in your car. You are re-writing how the "brain" in the computer talks to the hardware components, and also controlling some features of how the operating system behaves. -- Hope that helps the view as VCDS tweaks vs, ECU tune calibrations.