I remember using ud pullies on my mustang to slow down accessories to pick up a few more hp. not sure about how it would affect economy though.
metrompg did an experiment with taking the belt off his alt and picked up a lil gain in mileage, so i'd think it would be less than that since your still driving the accessories, but if you ud them all might be worth some gain.
they alternator should start putting out 14 volts at around 860rpm (revernced from civic shop manual), so as long as you were able to keep it above that, and reduced electrical load, it should be fine, altho it might bog it down a little at idle as it wouldn't be putting out as many amps, but then you have to ask your self why the belt drive system is set up how it is? I suspect that droping your electrical loads at the same time would be required.
I have one on my truck,which has a big v-8, single pulley, I did not see any difference in FE or power. I think they really only help HP at high rpms by reducing drag. Maybe on a small engine the difference would be more noticeable.