NO and Yes
Underdrive pully's do not lower the output of the alternator, but it does slow the alternator. That means that the alternator will not start to "Make Juice" until you give it a few RPM's. You may see your light go dim as a Pully'd car tends to run off of the battery at idle because the low RPM rate will keep the alternator out of the charge range. With a big system installed, that problem can definately be magnified. If you are going to run pully's, better to have a custom alternator built, and let the builder know what size pully's you will be running so that he can build the alternator with that in mind. that way the alternator can keep the batteries at peak at a lower RPM.