Hi, there's quite a few of us running 312 rotors on the polo's though they require different calipers and sometime carriers pending the caliper. The cheapest way to get to 312's is to source Audi TT or S3 Front Calipers. The result is very good. The stock hardware in 288mm can be made to be quite effective with the right pad choice too. Depends on what you're doing with the car.
You may have to trawl these build threads for more details, or someone may chirp up with more.
Louis19's Build Thread
Gavin's cheapy but getting more expensive GTI
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