It's a gimmick and does absolutely nothing to INCREASE power or voltage. All it does is relieve you of some cash from your pocket. Sure, your meter may read an increase in voltage, but its all smoke and mirrors. You can make your own with a $2 power diode from Radio Shack and some heats shrink tubing. Sorry to bust your bubble, but there's no magic bullet. You need a bigger alternator or less resistance in your wiring to make any real effect as noted.
edit: and YES the 15 volts is definitely going to screw some things up. If you read their (ZZP) disclaimer, it tells you that the stage II unit may cause over voltage problems. Lol...
