The stock GTI spindles are cast iron which means heavy, rusty, and even prone to cracking under heavy track use. Fortunately, there are a couple of OEM forged aluminum options that remedy all of the above. The Audi S3/VW Passat spindle is geometrically identical to the GTI spindle except for having a slightly shorter steering arm. The Audi TT spindle offers a lower lower ball joint mounting point to improve roll center which is great, but it it also has a much longer steering arm. The length of the steering arm directly effects how far the wheel is angled per amount of tie rod travel. The longer the arm, the less wheel angle you get for a given amount of tie rod travel.

The stock VW GTI arm has a 10cm steering arm. The forged aluminum Audi S3/VW Passat spindle has a shorter 9.6cm steering arm. Finally, the Audi TT has a much longer arm at 13.3cm. The plot below compares the resulting ratios.

As you can see, the GTI and S3/Passat spindles are similar with the S3/Passat being a bit quicker. It also shows how much slower the TT spindle makes the steering ratio. The TT suspension was designed as a total system. This shows mismatching components in a system can have cons you need to be aware of.