From my investigations (Google) it's generally acknowledged that the Z4 air intake is well designed and doesn't need any modification to get more air in. The contention seems to be down to the panel filter which is fitted. Obviously BMW supply a standard paper filter, and looking at mine I think it's worth changing it. I don't have any experience of after market panel filters as I've just used OEM previously (I fitted a Simota to my Z3 which was awesome!) Looking on eBay I have a choice of Pipercross, K&N, BMC all around the £30 to £40 price mark. Is it worth investing in a decent one, or just change the paper one more regularly? Mine is pretty mucky and the next service due is OIL Inspection, so I doubt it will get changed then.