I believe you are referring to Variable Speed Screw Compressor.
In General, lot of compressor brands claim VSD compressor can give energy saving anything from 5% to 35% range. If the plant air usage pattern is very erratic, the energy saving will be higher as compared to a stable usage pattern.
However, please note that VSD will not give you any savings if the VSD compressor is run as at full load. In fact, it will be worst comparing to fixed speed compressor (of similar motor size) due to conversion losses.
One more thing, make sure VSD compressor is located at well-ventilated area, as pro-longed high heat will cause pre-mature failure to the VSD panel.