Originally Posted by Jon D
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.
I have tried a tuning box for on my 120dM and the effect was massive. As far as I am told it leaves traces in the ECU, but I guess this would only be investigated by BMW if you have a warranty claim related to the engine, turbo, clutch, transmission etc.
I had a failure on a control arm for the turbo in my 120dM just after it went of warranty. BMW granted goodwill for most of the repair cost.