don't worry about what the birthsheet says, most zx1500.1's put out more than their birthsheet. I highly highly doubt that any zx1500.1 with a higher birthsheet you'll be able to hear a difference. I've had two zx1500.1's. The first one had a birthsheet of 1750 and the one i have in my car now has a birthsheet of 1550. I hear no difference at all.
^ what? to gain 3 db's you double the power in theory, to get from 1 watt to 2 watts would be 3db. so you could hear a difference of 1 watt. hahahahahahahahahahahahahahahahahaha