The manifold isn't restrictive for your engine, it has been pretty well designed, but they did also put a manifold with similar airflow characteristics on some bigger engines to restrict them.
Just putting a 325 manifold on can make things worse as the air comes down a nice large pipe, then hits a step into the smaller 320 head ports which is going to make a lot of turbulence.

In the same way a crowd would run better through a narrow passageway then they would through a slightly wider one with a step at the end.

 

ah ok thanks for the reply i see you`re point, i think i will leave the 325 inlet then and look for other things that could free up a bit of power,thanks again

 

You could try 325 twin cats and the exhaust, but the 320 has a pretty good sound all by itself which you would lose.

 

i agree with you saying the 320 has a nice sound from the exhaust! how come you dont get a loud induction roar from the 320 even with a k+n cone filter? my wife has a little corsa 1.4sri and that really roars with her induction kit.

 

BMW designed their airbox to be pretty quiet.
I believe its official name in "suction silencer".

The manifold too was made to be rigid to keep it quiet so they did plan from the outset to keep noise levels down.