There's a guy on here (forgot his name
) who has a K04 converted Octavia TFSI, he's done over 80,000 miles with the conversion (car is on over 120,000)
If you increase the maintenance (more regular oil changes), use Millers Nanodrive oil and only thrash the car when warm then it will do 200,000 miles
@RedRobin has over 110,000 miles on his K03 GTI - that's been tuned since around 20,000 miles IIRC