....I've only got experience of using the AP Racing BBK, but lots of it!!
I was involved (in a very minor capacity) in the final stage of development of the AP's for the Mk5 GTI and so AP themselves (their Senior Race Engineer) personally fitted my brake kit and we have had some long chats.
As a high percentage (85%?) of braking is on the front, you need not change the GTI's stock rear brakes. Also, if you do, you've some fairly tedious handbrake stuff to contend with. Also, largely due to the weight of the car, you don't need more than 4-pot calipers for fast road use and not even for trackdays. VW Racing have recently heavily modded a good friend of mine's Mk5 GTI (290 bhp and the rest!) and they independently recommended that his 4-pot AP Racing BBK (same as mine cos I introduced him to them) was totally adequate for trackdays. Also, 6-pot pads are fearfully expensive! The AP kit uses Ferodo 5200 pads and Dot 5.1 fluid and SS braided hoses.
People suggest mixing pads and discs etc to effect money savings but frankly I think it doesn't make sense to ignore what such specialists as AP have developed as a whole package - Do you really know better than these guys? I think not!
One of the criticisms of the GTI's stock brakes is that their pedal feel is not too great. Don't get me wrong, the stock brakes are very good but the AP's are waaaay better - You won't believe their stopping power and direct feel of the pedal response. I've had to use them in emergency and I've used them at 130 mph (GPS not speedo), in both dry and extremely wet conditions and I TRUST them. Obviously brakes are always very dependent on your tyres and road surface.
They fit both the Monza 18's and 17's but not the steel spare wheel. Each brake assembly weighs 10.6 Kg whereas the VW assembly weighs 15.10 Kg each front.
I'm not someone who uses his brakes that much - In fact when I was up at AP recently they inspected my brakes and asked if I ever used them! My style of driving is to try to smoothly arrive at the appropriate speed but on occasion I have totally outbraked others approaching roundabouts.
Without direct experience of comparisons I can't state that AP's are the best of all but I can say that I find it difficult to believe that there are any better brakes for the GTI. Equal perhaps but doubtfully better.
I'd rather have good brakes and not need them, than need them and not have them!
Hope this helps.
