Overdrive - may wear out bearings a bit faster (but we usually live at low RPM anyhow.) You'll get full output at a lower crankshaft speed, and you may increase total output of your alternator (but that is more determined by the voltage regulator and rectifier pack than by the pully size.) Will take more torque away from the rear wheels (just not sure how much) earlier in the power band if the electrical demands are high. Will spin your alternator fan slightly faster, allowing for more cooling at idle speeds (when alternator output should otherwise be low, reducing total stress on the solid-state electronics.)
Underdrive - will help improve bearing life slightly, but will lower the alternator potential output at various crankshaft speeds. Will reach full output at a higher crankshaft speed. Alternator will take slightly longer to cool at idle speeds when not at full output.
I would probably not suggest more than a 30% underdrive or a 12-15% overdrive for our alternators - more because of reliability concerns than for any reasons of output. Boy racers usually have component failures in town - we tend to have them about 100 miles from the nearest anything...