Originally Posted by gorban
Well BMW insurance is voided if you modify the car with a new map or a box for the engine control. Some tuning companies sell additional insurance.
In the EU, the car also becomes illegal because the manufacturer gets an EU wide approval for the car that includes the emission levels. Any tampering with the car and it loses this approval.
If you crash the car and the insurance company finds out about the remap you may get reduced or no compensation at all.
Still many owners do remap their cars. Most after the warranty is gone though.
This is why I didn't re-map my 330d and the same reasoning applies to my forthcoming 535d.
Having said that, I might eventually be tempted by a plug-in tuning box. These work by modifying the fuel pressure and turbo boost signals to the ECU, which then reacts accordingly by increasing both values. They are easily removable and, as far as I understand it, they leave no detectable electronic trace in the ECU; however, I willingly stand to be corrected on the latter point.
Another consideration is that the easy remove option is, of course, dependent on access to the engine bay; a front-end collision could preclude that and I imagine, unless previously declared, you'd then find yourself without insurance.