You don't need 6-pot on a Mk5 GTI unless you do a lot of track work.
Not necessarily true. 6-pot calipers enable a larger pad to be used, and also help apply a more even pressure over the entire pad. And don't forget, some mods are done on "looks" alone.

You don't need aftermarket rear brakes either
But matching front and rears do look better!
- Around 85% of braking is on the front on our cars.
That is not true. The EBD component of the ABS/ESP electronically varies the front to rear bias, and the rears, even under normal braking, do much more work. When the ESP activates, then
100% of the braking efficiency is applied to just one rear wheel only. Indeed, so much force can be applied to the rears, they can overheat and actually temporarilly disable the ESP - and this is clearly highlighted in the owners manual.
My understanding is that AP is an independent part of Brembo.
Again, we have discussed this before. AP are not, in any way related to or owned by Brembo, and nor have they ever been! The two are entirely separate companies, and have rival product ranges to boot!
AP make brakes/calipers/clutches for the Bugatti Veyron and the main F1 teams
They certainly do NOT make brakes for the Veyron. The Veyron, just like all high performance braking applications within the VAG stable (including the Bentley ceramics, Lamborghini, Audi RS cars, and the VW Touareg) all use Brembos. The more "standard" VAG cars use ATE calipers, and the pads are either Pagid or Ferodo.
As for F1 cars, AP do supply a couple of the "smaller" teams, but the vast majority, including the front running four teams all use Brembo.