My experience is for the polo as well.

I went from 2 1/4 to 2 1/2 inch, and the extra diameter made the low end and idle louder and helped the power a little. its not loud, I don't want people to noice my car from the exhaust (unless I'm pulling hard, then its the turbo they hear). From a lot of reading, 2.5" is good upto about 200kw (so for the little 1.8, no need to go bigger).

IMO, you don't need to pay for a branded one, just go to a recommended shop. Afterall its just a pipe.

For me the APR was about $1000, recommended shop did it for a third of that and looks clean.