The smaller pulley will turn the charger faster so the inlet pressure will be higher at all engine speeds the so if it was my car I would have it checked on a dyno before and after installation to see the power increase but importantly to ensure that the AFR's are safe.
Are you reducing from 72mm to 62mm if so this should be a drop in swap and gives another 30hp or so. If you get the engine mapped you might see 40-50hp increase.
For some pulley swaps you need a shorter belt, but in most cases where you have a "stepped" pulley the existing belt can be tensioned correctly. The shorter size is on the supercharger pulley side, not the engine pully side.
Make sure the fuel injectors are in good condition and that all the air hose connections are secure.