I've been researching brake kits. There are a bunch of options but it seem to be heavily dependent on subjective feel, how the car is being used, and cost of install/maintenance. 100% unnecessary for me? Yes. Wanted? Yes. I originally wanted to pull the trigger on a brake kit this month, but I decided that I needed to do more research first.
OEM TT-RS retrofit: 4 piston calipers with 370 mm rotors. This option works because of the platform cross fit from the A3/S3/RS3. We have the benefit of OE research and development with reasonable maintenance costs and proven design in all season drivability. New parts from OE are expensive, coming close to $4000 for everything, but if I can find the calipers and/or other components used with low mileage, it might make a very cost effective install option. Parts necessary include the calipers, front rotors, stainless brake lines, pads, new fluid. Some people saying that a master cylinder swap is necessary to retain a stock-like petal feel; so far that doesn't seem to be the case. If anyone in Ohio has done this swap, I would love to take a short test drive for pedal feel etc. Helpful links:
http://www.golfmkv.com/forums/showthread.php?t=175147
http://www.golfmkv.com/forums/showthread.php?t=176126
http://www.golfmkv.com/forums/showthread.php?t=149461
http://www.golfmkv.com/forums/showthread.php?t=136174
Ebay regularly has UK selling RS3 and TTRS front calipers for anywhere between $250-2000, depending on what they're selling with it. New, these parts would be close to $3000, so I'd say used calipers are the way to go. Still need to get the rest of the parts, including stainless brake lines, rotors, brackets if necessary. Also some control arm upgrades aren't very compatible, like the SuperPro retrofit that some people have used.
Brembo “Tier 3" $1800 - These are 4 piston calipers with 330mm 1 piece rotors. The cheapest Brembo option for the MKVI platform that serves it’s purpose. Street worthy with weekend track events are a high possibility here. The longest anyone I know or may know has had them installed is a little over one year - not enough time to really prove if longevity is going to be an issue. They're more than enough for daily and they are very civil. If I was thinking more reasonably, this would be my first choice, but as with buying mainly more expensive parts, I figure that if I'm doing it, I might as well do it all the way and not cheap out.
Brembo GT “Tier 1” - These are usually around $3995 and are extreme. Obviously they would work great, but for actual daily driving they might be a little much. However, with unlimited amounts of cash, I would spring for these in a second. 6 piston monoblock calipers, 355x32 mm two piece rotors. Pretty expensive to replace rotors, but at least we know Brembo is going to be around forever, certainly longer than I'll have my GTI even if I keep it for ten years.
VW Racing “Tier 2" - Though these are $2900 - on sale till the end of November for $2400, the replacement rotors are $1300 a pair. Even if that only happens once every few years, that is a pretty hefty daily driver pay to play fee. These are not up to the performance of the Brembo GTs, but far more than I would ever need, even tracking. These are six piston two piece calipers with 352x32mm two piece rotors. The nice thing about these are that they have VW specific hub spacers built into the rotor design, which means you don't have to add in other aftermarket pieces. These are only available from the UK, so shipping and replacement parts is a bit of a wait. Then again, this system is designed for VWs instead of being fit to all platforms, so there's a little more thought into the design, like the fact that the caliper design is specifically created for the GTI master cylinder. In terms of keeping it in the family, I could easily see myself going for these as well as VWR transmission and engine mounts.
Stoptech “Tier 3” - I really just have no interest in Stoptechs, but I do see that most people who upgrade their brake systems go with Stoptech. For the price and delivery of product, there's really no better product for the dollar.
17z Retrofit - I also don't want to deal with piecing together a brake kit from a platform that is so old. This tech is positively antique compared to the options we have off the shelf and upcoming. I'd even go for a MkVII brake retrofit over reaching back so far to piece together a kit like this.