For cruise we expect to see an increase of 10-15 KIAS over what your airframe did prior to the supercharger installation. Owners report 8-21, depending on the conditions.
The full potential of the supercharger should be realized from 12000' to mid teens. For example, our test Cirrus was fastest at 8000' in a cruise setting prior to the supercharger. After the FAT system was installed we saw the full 15+ knot increase at 12,000'. In addition to increased cruise speeds expect to see an increase in climb over what your airframe was capable of prior to the supercharger being installed.
A supercharged Cessna 182 will climb approximately 500 FPM faster at all altitudes above 5000'. A FAT powered Cirrus SR22 will climb from sea level to 12000' faster then any other Cirrus, over a minute faster then a turbo.
As noted in the Diamond forums, the supercharged DA 40 can pick up 25 knots in the mid teens.