I put mine in the stock position, right after the turbo.
I consisderd putting it underneath just after the flexi, as the factory V6 models have their cats here, but the heat shielding on the 1.8T is less and i didnt want to cook the propshaft CV joint so i just put it in the stock place. Plus as mark says, the flexi is already the lowest part under the car and often gets scraped already, putting an expensive cat down there to get smashed instead i decided wasnt a good idea.
The V6 cats are flat on the bottom and shaped to fit so they dont stick down under the car.
The sound of mine totally changed with the sports cat. It has a factory S4 exhaust fitted and with the stock cat sounded basically like a factory exhaust. With the sports cat and 3" downpipe it has a very prononced rumble, and rips nicely when pressing on, but is also properly silenced with no drone or annoying blare that you often get with performance exhausts.