Head work alone will achieve very little unless the port design is constrictive, not likely on this engine, plus you have to factor in any increase in airflow into the engine will require a matching increase in fuelling, so remapping would be required to make any usefull gains from the increased air flow.
A combination of increased valve lift/duration with different cams, increased compression ratio, a modified head and a matched exhaust will see improvements if you are prepared to reduce torque lower down and increase the RPM band, because that's pretty much the only way you can improve the output of a normally aspirated engine, but then you would have to look at changing gear ratio's to match the different torque band.
You often read of an extra 15bhp etc, but when your starting with 230bhp that's hardly noticeable for a lot of cash.
Two problems with this car is it's over geared and has a flywheel like a manhole cover, going to a lightweight solid flywheel and reducing the overall diff ratio will improve the throttle response and give a better speed range to the ratio's, the 3.46 diff is probably as low as you can go on a road car and still have a usable first gear, my E30 325I sport sprint car used a 3.67 diff with a direct top 5 speed box from a 323I, coupled with a 7500rpm limit and 220bhp it was a quick car in it's class.
But, the costs do not add up for day to day use, it would be cheaper to just buy a Z4M.
