Under drive crank pulley

Anyone ever tried this March under drive crank pulley?
Just changed my harmonic damper and found my stock pulley was rotted through one of the grooves. An accident waiting to happen.
Need to replace it but thought I might try this March pulley.
Will The under drive spin my water pump faster or slower?
http://ep.yimg.com/ay/chucker54/march-performance-billet-2-groove-crank-pulley-3.gif
http://chucker54.stores.yahoo.net/mapebi2grcrp.html

...Brian