I believe the biggest difference between the modes are the speeder response and how fast the car actually up and downshifts gear. ECO modes usually forces you in the highest gear possible to limit fuel consumption.
Normal mode is somewhat similar, but it will grant you that extra power when you choose so.
2/. If I do want to change the mode to Sport for that little bit of fizz (non techny term) is there any need to pop the DSG lever back a notch into S at the same time as selecting Sport Mode via the console buttons.
My personal experience in my current 220D BMW is that whenever you switches modes there will be a slight delay. So if you want to have that instant power, I would choose use the padde shifter while changing mode.
When i test drove the GTI I noticed that you don't have to go to manual mode in order to do so. All you have to do is to hold the paddle shifter, and it will go into manual by itself and will remain in manual.
I am by no mean an expert so right me if I am wrong
![Smile :)](./images/smilies/icon_smile.gif)
I'll be recieving my own GTI next month so i'll be able to edit this if i discover something else.