How does it affect the Engine Management and other systems of the car's performance?

 

The simple answer, badly, you should always have a good electrical systems, good grounds, a well charged battery and an alternator that puts out proper power. All aftermarket systems have battery voltage compensation maps what allow for corrections for low or high voltage, since the same pulsation of an injector at 2 different voltage would produce differences in fuel delivery, this applies for spark power as well.

Not to mention that the haltechs dont like power drops of bellow 11-12V, specially while cranking.