The sure way to find out what is happening with the OEM air-intake is to hook-up a datalogger and set it to simultaneously record: Speed, RPM, air-flow at the MAF and air intake temperature. You then do an acceleration run from low revs to the redline at WOT (Wide open throttle - foot to the floor). From the information you record, you can calculate the Volumetric Efficiency of your engine.
I have a DashDynoSPD datalogger:
http://www.auterraweb.com/dashdynoseries.html
and save the recorded files as multipoint comma separated values (.csv) which I can then open in Microsoft Excel. I've made an Excel Template file which I can then insert the .csv data and from this I can then produce a graph of all my experiments with air-intake mods.
I have a BMW Z3 M Coupe which I've been modding for the past three years and I've already performed my own mods to the OEM air-box and exhaust all of which have cost me NOTHING, because they're just "tweaks" to what the BMW engineers have designed. The air intake does genuinely operate with ram-effect. I have recently designed another intake mod and I now get at 100% Volumetric Efficiency or above at speeds of around 60mph, whereas before, this occurred at around 80mph. The graph below shows the data from my car under acceleration (although in this instance I didn't take the engine to max revs in any of the gears).
The graph below is prepared from the same data as the graph above, but I have removed the "spikes" to produce a "smoothed" representation of the change in Volumetric Efficiency with increase in speed. From this data, it appears that a Volumetric Efficiency of around 110%-115% might be attainable at the speed limit of 155mph.
I would therefore suggest that the E90 M3 air intake will be working extremely well in OEM format.