3hp would be 5.4%!
As oilpan noted though, his pumps pull less than 100W, so only a fraction of a horsepower.
I'm really not sure what kind of gains I'll see, there are a lot of variables here. The closest thing I can compare it to is an alternator delete. The Metro only uses 128W to power itself while idling according to MetroMPG's testing. But, that is exaggerated by the alternator's inefficiency, so its probably closer to 250W of engine power. Say the water pump uses 80W, thats 1/4th the gain. MetroMPG saw a 10% gain from de-belting his alternator (but kept a belt on his a water pump). I'll be debelting everything. So, my guess at this point is probably 3-4%.