The main springs have weakened or are simply too short Peter, assuming you have pushed the car back and forth a short distance to find an average static position. Dampers will not affect this situation unless they have seized or are too stiff to allow normal suspension articulation. Given your mention of competition use the car will have had a hard life which may have contributed to this situation.
With that amount of play you will have little roll resistance and a large amount of rebound suspension travel over bumps.
Suggest the well proven
SSL RS kit which includes full set of dual rate springs, adjustable gas dampers and steering bearings. It allows accurate setting up of the ideal zero static play on rebound spring you mention.
