Hi Tom,There are two options the cheaper way is to replace the exhaust system and air filter unit for uprated ones,this will only give you small gains.
Or you can super charge or turbo charge the engine this will give you big power gains.
around the 300bhp.
Have you got sat nav fitted as standard.?
You can get radar detectors fitted.
I would look in magazines like performance BMW,BMW car magazine and total BMW magazine.These mags can show you some good gadgets..
Hope this helps. Thanks,mark
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BMW Technician
Senior BMW technician,NVQ 3 CITY GUILDS 3 RTITB Level 3