You could remap the GTD, and keep the money you would have paid for a GTI, have a few holidays, enjoy yourself and still have some money if anything goes wrong and VW won't cover it. £5k extra is alot of money !!
Remaps nowadays are very good and to have a fail on your car within 3 years warranted probably won't be caused by the remap if you get it done by a reputable dealer. If a part fails it was probably going to fail anyway.
But at the end of the day it is up to you.
