Originally Posted by Boss330
BMW might be able to detect unusual output signals from the ECU. Ie if the voltage to the fuel pump normally is 11V to maintain normal fuel pressure and on your car the log shows that it's repeatedly running at 12V there might be some issues with BMW (just a very crude illustration).
Performance gains are possible as long as boost and fuel is higher, but these powerboxes have often been over advertised.
The drawbacks are obviously that the ECU are still operating at it's std. maps and believes that the boost pressure still is std. The ECU maps are not altered to advance/retard ignition etc to the higher boost and fuel pressure. Meaning that the map is still for a 560HP engine setup with a given boost pressure and not remapped for higher boost and HP. This can lead to problems with lean/rich mixture, exhaust temperatures etc.
Ahh finally a voice of reason
You're correct BMW can detect changes in signals, temperatures, revs, altitude etc
and if they don't correlated in a reasonable way they likely will log an fault . Not all faults are stored in a visible fashion so BMW can also retrieve hidden faults the aftermarket code readers/erasers won't see. That's why some of the code readers/erasers will flood the ECU with false/benign codes (such as exhaust flap) to (hope) erase any other codes
In short if it would be that easy to come up with a simple piggy back tune (or any other) the bigger tuners would have done so already.
As now has been demonstrated over and over again, there's no magic 8 ball to tune the M5 and BMW will certainly make sure to keep tuners at bay and introduce new and improved measures. The honest tuners providing tunes today will tell you that they can modify the ECU/maps but it will also void your engine / drive train warranty.
It all depends how desperate you are and how much of a risk you're willing to take.