Been on Euro carparts webbo, and they only seem to have 288mm diameter discs or 345mm, I thought these were 312mm for boggo standard on the front?
The ECP site is notoriously tricky when trying to select parts especially discs and pads as it will list all the available mk5 golf sizes rather than those for the gti,
Correct size for the front of the gti is 312x25mm and the rear is 286mm, bear in mind that the rear disc spec has been changed at dealer level to now be 282mm but they will fit perfectly fine
If buying from ECP then I'd personally avoid any brands other than Pagid or ATE as these are OE quality equivalents, don't rule the dealer / tps out completely as imo the oem discs are of better quality than most 3rd party stuff...
As for pads, ferodo ds performance are not a bad choice if you don't intend any track work and have the benefit of being reg 90 compliant and therefore 'road legal', ds2500 is also a good pad but getting pricey and maybe overkill for your intended use?
Mintex m1144 are also a good choice and offer the same level of performance as the ds2500 at around 60% of the price, imo I wouldn't spend big money on rear pads as it's just not worth it and oem ones will be perfectly fine, if you do want to then consider the Hawk HPS ones as they are cheap and very good