I tow an Elddis (MTPLM 1150Kg) with a Mk4 Golf estate TDI130, with no problems whatsoever. Moreover, it returns between 30 and 35 mpg towing, and an overall 54 mpg solo (no heavy traffic or heavy right boot.)
This has the same engine as the Mk4 GTI TDI130, and therefore (almost) exactly the same performance. If you do not need the 'street cred.' of the GTI, you can make a significant saving on insurance by going for the estate (group 8 vs group 15?), and improve your luggage capacity.
Hope this helps,