Many VW/Audi/Skoda etc parts are interchangeable - so have you looked into the possibility of fitting the 312mm disks and callipers from a MK5/6 GTi on?
Plenty of Golf GTI owners do a BBK when they upgrade the power so theres usually a stock Golf GTI front brake setup for sale. (infact there was someone selling GTI callipers just yesterday).
Also.....
Tarox Front Big Brake Kit Volkswagen Polo 6r
fabiavRS/Cupra/Polo GTI Brake upgrades | VAGOC.co.uk
Polo 6R braking thread (Mainly GTI/Diesel)
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