apart from anything else, there were as set of measurements done some years ago by one of the more technical German A2Club members.
They measured with different filters the amount of fuel and air available, including the standard ECU mapping, and found that filters made no difference to the power available as the car understood it.
Now, your personal mileage may vary. However, the setup as it is is not as bad as it could be and, to be honest, it can't be that bad, otherwise 70mpg wouldn't be possible.
I can find the links for you if you want, but if you want more power, go for a remap (Stealth) and see what they can get out of the car. There isn't much (if anything) to be gained from the inlet tract.
Bret