Because you are in Europe I'm assuming you have the Euro version with catless headers. In this case, replacing section 1 of the exhaust with a catless system will yield the most gains, but make your Z4 illegal at the same time. Other than that, check out reflashes from AA, Bren Tuning, ESS, etc. Typically the limit on these engines isn't the heat they produce, so I wouldn't worry about an uprated cooling system.
You might get a combined 15 hp out of a tune and section 1 cat delete.
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