Leak down is good if you keep a track of it between races for example and if you see a large increase you got problems. Leak down is only an indication as most rings top rings need combustion pressures to seal more effectively. The pressures that you check during a leak down are nowhere near the operating pressures so you could be getting false results . I have experienced a case where an engine had 8% leak down dissasembled the engine fitted a sleave then had 20% leak down but car went quicker straight off the trailer!!!