Contrary to the statement from the OP "why would you buy a GTI and worry about MPG", why would you buy a GTI if you were not considering all of the running costs to a certain extent?
There are many premium badged cars that are a lot faster and more performance oriented, for circa £30k especially if you will have say a 2 year old one.
The GTI is supposed to appeal to the masses because it ticks all the boxes and the average person can justify having one whilst still enjoying hoofing it around whenever suits.
The MPG factor is definitely a selling point to most people I would imagine if they were considering a GTI against the competition.
