The Audi Airfilters are excellent as are their I/C's for achieving the factory figures in all environments. If you want to turn up the wick, there are many aftermarket products available and its a mine field. Even MRC say an upgraded foam filter in a standard airbox works as good as anything they've seen.
Others swear by what they've just laid out for to justify the, often, obscene price they paid for it. I know the Armaspeed air intake works ok, looks great and MOST Importantly amplifies the wastegate chatter when you lift off