What about a standard m50 manifold swap?
Already done that :-)
+K&N Filter and CAI
https://www.youtube.com/watch?v=fARuk-nx3Pw
Go to about 53 seconds for relative improvements
Just watched it again and realised I didn't test after the Inlet swap. Might try that today also. If the private road is still available
They quote :
Z3 2.8 24V Original 193 hp Chipped 205 hp Original 280 NM Chipped 310 NM
Z3 3.0 24V Original 231 hp Chipped 245 hp Original 300 NM Chipped 325 NM
I will be heading down shortly to talk to them. What sort of price should I be looking for?