Pilot DAR provides an excellent analysis of STOL kits. You should not think of them as enabling shorter field take-off and landing as the name suggests. They do provide enhanced handling qualities at speeds right down to the stall so enabling the pilot to feel more comfortable, but the stall can then come with a sudden unexpected break. The STOL kit should not be understood as reducing the stall speed to enable shorter landings than those provided in the aircraft manual.